I will say do the Coke-Cola tour. It is a lot of fun. Show you the history of cokes, and even you taste most of Coke-Cola Brands all over the world. Drink as much as you want to, but should not take over 3 hours maybe even less, unless there is a long wait. After you get done there, just walk over to the underground and do some shopping.
Or
If you are young, and like the heat, and like coasters you can do Six Flags Over Georgia. It is only 15 minutes west of ATL. You will spend the entire day there.