I grew up in Atlanta and lived there well into my 20's. It's an amazing city!
Some things you should check out:
The World of Coke: Atlanta is the birthplace of Coca-cola. You can visit the museum, which is filled with tons of fun activities. The best part? You can taste 100+ coke products from different countries around the world. (Cost: $12-14 for adults)
Atlanta Aquarium: One of the more recently built large aquariums, Atlanta really poured a lot of money and effort into this one. See everything from otters to beluga whales.. even pet sharks! (Cost: pretty pricey, $40-50/adult)
*Pro-tip: Coke and the Aquarium are in the same plaza.. buy a joint ticket to save $$.*
Centennial Park: Lots of fun to play in the fountains in the Hotlanta 100+ degree summers, and beautiful to walk around in the the wintertime. They decorate beautifully for the holidays.
Underground Atlanta: Pretty touristy these days but still with some amazing views and architecture, Underground Atlanta is a must-visit for guests. You can walk through the old viaducts that remained after the original structure of one of the oldest buildings in Atlanta burnt down. (Cost: free to explore, but allot money for shopping)
Little Five Points: Originally created as a shopping center/commercial area, Little Five Points has now become the hub for Atlanta's (maybe even the near South's) alternative culture. Looking for some hippie culture? Listen to their indie radio station or visit some of their many music festivals. (Cost: shops/restaurants are slightly pricey)
*Pro-tip: While you are there, visit the Vortex for lunch! They are known for their burgers and atmosphere!*
Stone Mountain Park: Fun for the entire family, you can hike or even stay the night to watch a huge laser show projected on the side of the mountain!
Fox Theater: Catch a performance at the historic Fox Theater during your visit!
Braves Game: Atlanta loves their baseball. If you visit in the summer, grab your friends and tailgating equipment and head down to Turner Field!