Our trains are very simple (too simple for my taste, they don't go enough places). Along the train lines you could visit the High Museum of Art (about $20), the Aquarium (which is huuuuuge) is easy to get to but is only open past 5 on certain nights ($20). Food and shopping are easy to get to. If you happen to be in town on the night of an Atlanta Braves game, tickets can be very cheap.

While not right inside the city of Atlanta, downtown Decatur is on the train line and has a nice collection of shops and restaurants. If you want to see the big buildings, downtown proper isn't too interesting outside of business hours but Buckhead is good for shopping/browsing and a meal.