One of the most common questions on the Atlanta forum on TA is "What should I do during my <N> hour layover at the Atlanta airport?" A search of the forum will turn up plenty of threads with suggestions.

At the airport 

 In general, if your layover is less than three hours, you shouldn't even think about leaving the airport -- the chances of missing your connecting flight are just too great. Most of the more popular attractions in Atlanta are far enough away that it's going to take you some time to get there and back, and you'll need to allow time to get back through security and get to your gate.

 If you're staying at the airport, you can find complete listings of the shops, restaurants, and services in each concourse and in the terminal building at http://www.atlanta-airport.com/Defaul... . It is not necessary to go through security again to pass between any of the concourses (T, A, B, C, D, E), so you're not limited to the options in the concourses for your arriving and departing flights. For shops/restaurants in the terminal area, however, you will have to exit the secure area and re-enter, so factor time for that into your planning. 

Between 6 am and about 9 pm, nearly all of the shops and restaurants are open. After 9 pm, things begin to close down for the night, and after 11 pm there's not much open on the concourses (particularly C and D). Atlanta Bread Company, Wendy's, and Dominos Pizza are open 24 hours in the Atrium area of the terminal (outside security). There aren't, however, any newstands or other shops selling sundries after 11 pm, so if you need something to read, an aspirin, a travel pillow, or a souvenir t-shirt, get it before they close or wait until morning. 

 None of the dining options in the airport are particular noteworthy -- there's typically the same sort of chain eateries you'd find at most other airports, though there are a lot of them and the sheer volume means more variety than a lot of other airports, especially since you're not constrained to one concourse. Menus tend to be somewhat limited because of the constraints on having any sort of cutlery that will actually cut anything inside the secure area. For local flavor, the best option is probably one of the Paschal's locations (Atrium, Concourse A food court, Concourse C centerpoint ) -- the food is typical southern meat-and-three steamtable fare, with decent fried chicken, blackeyed peas, yams, etc.

Most of the concourses have some sort of larger book/magazine store near the center where the escalators from the transportation mall are located -- this tends to be where most of the stores selling larger/higher-dollar items are located as well.  

If the boredom of hanging out at the airport gets too intense, you can try to while away the time with one of the displays of art and other artifacts in various spots. In the transportation mall between Concourse A and the T-Gates, there's a permanent installation of about 20 stone sculptures from Zimbabwe, displayed between the moving sidewalks so that you can take in a full 360-degree view of each. There are also displays of art created by Atlanta youth through the airport, as well as more conventional art displays in other locations, including the T Gates and the International Concourse (Concourse E). 

Leaving the airport 

If you have more time, and decide you want to leave the airport, have a plan. Know how much time you have to work with, and make sure your plan includes some contingency time to cope with unanticipated delays -- your airline's not going to be very sympathetic if you miss your outbound flight. If you need to store bags/luggage, the only places at the airport to do so are the Wrap-A-Bag locations in the North and South Terminal areas near baggage claim. Note that while there are different hours for each location, bags stored at the North Terminal location can be picked up from the South Terminal location -- essentially, the hours are 6 am to 11 pm. Rates start around $5/bag for standard items and go up for bigger pieces.

 Before you plan to leave the airport, consider the time of day and the current wait times for the security lines. At most times, the security lines move quickly, rarely taking more than 10-15 minutes to get from the end of the line through the security checkpoint. But at particularly busy times, they can be much longer, taking an hour or more (2.5 hours is about the absolute worst case, but that rarely happens). Keep in mind also that even once you're through security, you still have to get to the gate, which at ATL can be as much as a mile away, and can take 10-15 minutes for far-flung gates on D or E concourses. As a guideline, if the security lines don't reach the Atrium area, it's probably less than 20 minutes to get through. As long as they're in the Atrium area, it'll probably be 30-40 minutes. Past the Atrium into the arrivals area, 40-50 minutes. Past the arrivals area and into the halls between baggage claim and the Atrium, an hour or more. All the way into the baggage claim areas, figure at least an hour and half.

Your options for venturing outside the airport boil down to cab, MARTA, and rental car. One thing you absolutely must consider is that while getting a cab from the airport to practically anywhere else is no problem at all, getting a cab back could be a challenge -- hailing a cab on the street, even from major tourist attractions, is pretty hit-and-miss (mostly miss). If you take a cab out and plan to take one back, you should get the number of the company, make arrangements to have someone pick you up again at an appointed time, and leave at least an extra hour for them not to show up and for you to call and nag them. If missing your flight is going to cause a hardship, you'd be well advised NOT to rely on a cab to get back. 

That leaves two other options: MARTA, and a rental car.  The MARTA train system will get you within easy walking distance of many of the major attractions, relatively painlessly. If you're going to downtown attractions, Midtown, Buckhead, Perimeter Mall area, or downtown Decatur, MARTA's a good option. Trains leave the Airport station about every 10 minutes.

• There's only one platform, but there are two different lines operating on the same tracks. One goes from the Airport to North Springs station in Sandy Springs, stopping at all stations from the Airport to Lindbergh, then branching off to Buckhead, Medical Center, Dunwoody, Sandy Springs, and North Springs. The other runs the same route to Lindbergh station, then branches northeast to Lenox, Brookhaven, Chamblee, and Doraville.  For any destination in Downtown or Midtown, it doesn't matter which train you take.
• Time to downtown (Five Points or Peachtree Center) from the airport is about 20 minutes, to Midtown or Arts Center stations about 25 minutes, 30 minutes to Lindbergh, 35 minutes to Buckhead or Lenox, and about 45 minutes to the end of either line. In your planning, factor in the 10 minutes between trains. Note that the last train is around 1 am most days, and that if you're going to Buckhead, Dunwoody, Perimter Mall, or points north (or if you're coming back from there), you'll probably have to switch trains at Lindbergh Station after 9 pm,  which will add a bit to the time required.
• For Little Five Points or Downtown Decatur, you'll want to transfer to the East line at Five Points station. Schedules are similar to the North-South lines -- figure on about 35-40 minutes to get to the Decatur station. 

If your intended destination isn't within walking distance of a train station, you should probably resist the temptation to rely on the buses to get you the rest of the way. Bus routes in Atlanta are meandering and confusing, service is infrequent (no more than once or twice an hour on many routes), and schedules are notoriously variable. Unless you've built an extra couple of hours into your planning, you shouldn't depend on the buses to get you to your train on time. MARTA's online TripPlanner can help you figure things out if you decide to ignore this advice.  Alternatively, Google maps now lists MARTA transit information between various points.

Driving is the worst way to get around Atlanta, except for all the other ways.  If you want to go someplace that isn't near a MARTA station, a rental car is the most reliable option. There are plenty of rental car companies at the airport -- the cheapest tend to have pickup/dropoff locations located off the airport property, mostly along Camp Creek Parkway. Traffic between the airport and downtown tends to be heavy during the morning rush hour period (from about 7 am to 9:30) so allow plenty of time. Traffic anywhere in downtown and along I-75/I-85 south back toward the airport will also be heavy during the afternoon. Check the Georgia Department of Transportation's Georgia Navigator site before heading out for current traffic/construction conditions. Keep in mind that for any rental car, you'll need to allow time to pickup/return and to take the shuttle bus between the rental car facility and the terminal (none of the rental car lots are located at the terminal).  Generally speaking, with a layover less than 4 hours, you probably won't have time do much of anything once you deal with the logistics of getting and returning the car.

 Attractions

For Downtown attractions like the Georgia Aquarium, World of Coca-Cola, CNN Center, etc., you should figure on at least an hour and a half to get there and back via MARTA/walking. Driving and parking would generally be comparable -- it'll take less time to get there, in most cases, but you'll also have to deal with parking and then walking to the venue. For a four-hour layover, planning to be back at the airport at least an hour before your outbound flight, that means only a half-hour of time to spend enjoying yourself. For points farther from the airport (Margaret Mitchell House, High Museum of Art, Botanical Garden, etc.), you'd have even less time.  

 If you simply want to see some of Atlanta, one option would be to take MARTA to the North Avenue station, walk a block east to Peachtree Street, and start walking north. This will take you past the Fox Theatre and through the thriving Midtown shopping/dining/entertainment district. You can get a train back to the airport from either the Midtown Station (between West Peachtree and Peachtree near 10th Street, a little under a mile) or Arts Center Station (on West Peachtree just north of 14th Street, about 1.5 miles). Note that if you are driving, you'll pay to park (assuming you can find a place) almost anywhere in this part of town.

 For shopping, the easiest option would be to take the Northeast MARTA line to Lenox, which would put you next to the Lenox Square mall, Atlanta's flagship mall destination for the last 40 years. Despite its age, it's been frequently renovated through the years and is more up to date than many malls built in the last decade. On the opposite corner from Lenox Square is Phipps Plaza, which a somewhat smaller, somewhat more upscale cousin to it (Tiffany, Lord and Taylor, Saks, etc.). The same management company operates both and provides a free shuttle service between them. There's also a free shuttle service, the BUC, throughout the Buckhead retail/hotel/dining district that makes it relatively easy to get around its service area at peak times on weekdays; it doesn't run between 9:30 and 11:30 am, between 1:30 and 3:30 pm, or after 7 pm. Saturday service is from 10 am to 9 pm, every 30 minutes. There's no BUC service on Sundays.

 Unless you have the better part of a day to spend, don't try to take on Zoo Atlanta, Oakland Cemetary, the MLK Historic District, the Atlanta History Center, Atlanta Botanical Garden, Piedmont Park, Fernbank Museum of Natural History, etc. They're not readily accessible from the train, and most will require more time than all but the the lengthiest layovers.

Dining

As with attractions, the Downtown, Midtown, and Buckhead areas within walking distance of MARTA are going to be the best options. There aren't a lot of particularly attractive options close to the airport, though a few probably merit special mention. There are two somewhat upscale restaurants close to the College Park MARTA station (first stop after leaving the airport, close enough for a cab ride despite the advice above): The Feed Store and The Pecan.

The Manchester Arms (a British-style pub) on Virginia Avenue also gets a lot of positive reviews, but isn't really an easy walk from the MARTA station -- if you decide that's more your speed, a cab is probably in order. Finally, no mention of the food options near the airport would be complete without referring to the Dwarf House -- Chik-Fil-A founder Truett Cathy's original restaurant, where it all began. Located on Central Avenue in Hapeville (also Jeff Foxworthy's hometown), just across the street from the Ford Motor Company factory, the Dwarf House is open 24 hours and offers both a full table-service menu (including hamburgers, though apparently no one told the iconic Cows) and the more familar fast-food counter service Chik-Fil-A menu. Not accessible by MARTA, though, so either get a car or take a cab (and plan to call for a pickup when you're done).