I was going to say Atlanta too. Of course, I am partial since I live here! You can visit the Coca Cola museum and the Georgia Aquarium which are next door to each other. You can get a City Pass and visit several attractions for one price. Lots of good restaurants, go to a Braves game (although they stink right now), visit Centennial Park, the zoo, Stone Mountain (about 30 minutes away). Not sure if you are into shows but the Fox Theatre is beautiful and a great place to see shows.
I also second everyone's Chicago suggestion. What a fun place! So much to do and see. Have some good shoes on tho.