For any first time visitor to Boston, there are so many things you'll want to do and see. When you are visiting in the spring, summer or fall, be sure to plan a historic walking tour of Boston known as the Freedom Trail. If you don't want to do the whole thing, take it in pieces, or do it over several days.
You'll have to plan ahead if you want a fun tour of the city by land and water, known as the Duck Tours. This one is great for kids and adults alike. Educational and entertaining. They are very popular so call ahead to book your tickets. Another option to get a narrated spin through town are the many trolley tours. The Beantown trolley is one of the most popular and as with pretty much any of the trolley tours has the added benefit of letting you get on and off along the route.
There are many fantastic museums for both adults and kids. The Museum of Fine Art, The Museum of Science (take the kids to this one!!), The New England Aquarium, and many more. Save these in case you get a rainy day while you're visiting. If going to the aquarium, especially on rainy days and weekends, get there early. The ramp which forms the primary way of visiting the aquarium gets unpleasantly crowded. The Childrens Museum is a big hit with kids -- although parent gripe about the lack of decent parking and the crowding (which never seems to both the kids!)
During the baseball season, you're in Red Sox Nation! Head over to Kenmore Square to catch the game at historic Fenway Park. If you can't get tickets, stop by any of the local watering holes for a drink and cheer along, you won't be alone. If you are a Yankees fan, probably best to keep it to yourself.
Go for a walk through Faneuil Hall Marketplace for some buzzing activity and people watching. There will be street performers and plenty of food options if you're hungry. Countless bars will wet your appetite if you head over later in the day. Just a bit further along is the city's North End, with the best Italian food and some of the best seafood in the city. Don't even thinking about parking though -- use the overpriced Government Center Garage or walk from one of the many nearby T stops.
For less touristy fare than Faneuil Hall and the city's top shopping, head for Newbury street. Summer or Winter, this street will delight shoppers and foodies alike with its boutiques and wide variety of restaurants. It starts right at the Public Garden and proceeds for the length of the Back Bay neighborhood.
If antiques are more your thing, Charles Street has some of the best antique stores in Boston as well as a number of fun restaurants. Charles Street is long enough to keep you occupied, but no more than a reasonable stroll. But leave the kids -- definitely more of a grown-up scene. Start with a coffee at Starbucks at the Common end and finish with a drink in The Clink (Liberty Hotel) at the other end.
Charlestown is just across the Charles River from the North End. It does not get a ton of tourists, although has a couple of worth sights in the Charlestown Navy Yard (home to the U.S.S. Constitution, aka "Old Ironsides") and Bunker Hill. Both are worth a visit for history buffs and the area around Bunker Hill has a number of quaint streets.
Cambridge is synonymous for many with Harvard University, but that's only a small corner of this expansive city across the Charles River from Boston. Some of the funkier restaurants and bars cluster around the various "squares" in Cambridge. The Squares are really overgrown intersections, but are handy reference points and typically have a Red line T stop, making them easy to reach from Boston if you want to sample one of the many Brazilian restaurants or authentic Boston Irish pubs. Cambridge also features MIT and both MIT and Harvard have interesting museums. Harvard Square has become depressingly bland and only needs to be visited if you want to see the University or buy a sweatshirt at the Coop. It used to boast a eclectic collection of eating, drinking, and shopping but in the past decade has been overrun with banks and the same chains that can be found anywhere else in America. A few of the old denizens still remain: for example Charlies Kitchen on Eliot Street makes a mean burger and has its own unique vibe.
Charlies is not really a kids place. Not that its fancy, but more because its a relatively hard drinking clientele. If you are looking for kid friendly dinning during your visit, Bertucci's is a safe bet with a number of locations in and around Boston. The pizza is pretty good and they arm each kid with coloring and some dough to play with. Legal Seafoodsis surprisingly kid friendly, although many find it a bit expensive option for the whole family. US visitors will find plenty of familiar options if neither of the former are conveniently located. And yes, there are Dunkin Doughnuts pretty much everywhere you turn.
If you are planning on taking in the sights, you might want to consider a Go Boston Card or a Boston Explorer Pass. The former is good if you intend a few days of intensive sight seeing as it gets you in to pretty much everything for a single, fixed price. The Explorer Pass lets you pick you choice of any 3 things -- again for a single fixed price and you have 30 days to use it -- so a good option if sightseeing is only part of your reason for visiting Boston. Both options also have skip the line privileges at some of the busier sights.
Don't stress about driving, Boston is fairly compact, so you can walk to many of the things you want to do or just take the T. It's affordable and easy to get around the city! Note, the last train runs at 12:45 A.M. Cabs can be a bit expensive for getting around the city so use sparingly if you're watching your budget. This applies for getting into town from the airport as well. You have a couple of relatively easy options: the "Silver Line" (it is actually a bus, but ends up on its own underground route, so it goes quickly). The Silver Line will get you to South Station -- the main (and modern) Boston transportation hub and right next to downtown and provides access to subway lines as well as commuter rail and Amtrak to points farther afield. A second option for getting into town from Logan Airport is to grab the airport bus to the subway stop -- the bus is frequent and has you there in 5 minutes. Downside is getting your gear on and off the free bus and then transferring to the subway. But its worth it if you want to save money and it puts you right on the Blue Line which will take you into central Boston after just a couple of stops.
Regardless of what you do, you will enjoy your stay in quaint little Boston.