When in the summer are you going to be there? My girlfriend lives near Columbus and I visit there from MI every other week. It can get pretty hot down there at times.
I would second the National Infantry Museum. I haven't been there but I'd like to see the National Civil War Naval Museum, which is also in Columbus. I do want to see Andersonville too.
Another option if you're outdoorsy is Providence Canyon State Park, near Lumpkin. There's a nice little hike down there, but again you might want to think twice if it's hot, and watch out for snakes. Lumpkin also has Historic Westville, but I'm not sure what their hours are.
If you're into wildlife/birding, there is the Eufaula National Wildlife Refuge near Eufaula, AL. There might not be a lot of birds around in the middle of summer, though.