As far as renting a car at the airport, all of the car rental facilities are off-site, however they are very close to the airport.   When you exit the airport, head to the bottom level.  If you have status with your rental company (such as National Emerald Aisle or Hertz Gold) you can board a bus as you exit at the 2nd curb.  If you do not, all rental windows are located at the bottom level near baggage claim.  The shuttle to the rental lots takes approximately 3-5 minutes depending on traffic.   Finding the drop-off for rental cars is very easy when returning to the airport, as there are a lot of signs, and really the airport is medium sized so you will not get lost.  Even if you miss the turn it only takes minutes to turn around (unlike Dallas, DC, etc.).

One word of caution, there are no gas stations if you are traveling from I-77 to Billy Graham Parkway.  I am unsure about 85 as I don’t travel from that direction. You will want to get off on an exit before Billy Graham (you can get off at Woodlawn and go to the right).  Then all you do from the gas station is make a left and you will be on Woodlawn, which turns into Billy Graham and on your way back to the airport.  

If you are going to travel while in Charlotte, you need a car.  Mass transit is virtually non-existant.   There is really not a lot to do downtown and if you traveling on business and not staying in the city, it is a necessity.