Unfortunately for most tourists, Atlanta's subway stations (MARTA, which locals refer to as "the train") aren't particulary close to the more interesting parts of town. Atlanta is incredibly spread out, and MARTA basically makes a big "X" through the center of the city. While you can transfer to one of MARTA's many buses, the routes are complicated for the uninitiated. MARTA is a great option for getting to and from the airport, though. Decatur (one of the few exceptions) is a wonderful place to eat drink and its MARTA station is right in the heart of a charming town square. MARTA has recently added 2 routes geared to visitors. The Downtown and Midtown Tourist Loops stop at most of Atlanta's attractions including the Georgia Aquarium, High Museum of Art, CNN Center, Georgia Dome/Philips Arena, Martin Luther King, Jr. Center, Atlanta Botanical Garden, Underground Atlanta, State Capitol, Fox Theatre, Piedmont Park, and the World of Coca-Cola. They have also started a "Visitor Pass" that is good for multiple days.
Cabs are easy to find at the airport, but you shouldn't depend on them to get you to and from restaurants and museums. On busy nights, they might be available in the more crowded parts of town, but it's a good idea to keep the number of a recommended taxi company and have them dispatched.
Walking is easy enough in Atlanta, but at times you'll feel like the only one on the sidewalk. Some parts of town are more pedestrian friendly than others (Decatur, Midtown, Downtown, Buckhead) with plenty of signals and crosswalks. Don't plan on cars giving pedestrians the right-of-way. Atlantans love their cars, for sure, and so few drivers ever walk that they don't watch for people like in other "walking" cities. The great irony is that the places in town that are among the best for strolling and site-seeing (like Virginia-Highlands) require a car to get there.
Most Atlantan's wouldn't dream of depending exclusively on public transportation to get around, and after a few days in town you may agree. There are so many great little 'pockets' to be discovered, yet very few are walking distance from most hotels, or from one to the other. Don't expect to be able to walk from downtown into nice little shopping districts, there are just too many blocks to cover on foot. An editor here works in the heart of Buckhead, and is often giving walking directions to disappointed tourists wondering where the fun is. Renting a car would make a huge difference in finding the charm that simply doesn't exsist in any one neighborhood. But get ready to get lost. There's no logical grid system or simple numbering of streets. Get a good visitor map and depend on southern hospitality and long-winded directions. Yes, there are dozens of streets named Peachtree, but you know when you're on the main drag. It runs through the heart of the city. It's crowded. It's slow. But it does allow you to see some great parts of the city from Downtown to Buckhead.
