After the air museum in Warner Robins, Macon has a couple interesting places - Ocmulgee Mounds National Historical Park and the Allman Brothers Band Museum. Maybe overnight there and then swing west towards Columbus for the Little White House and/or Callaway Gardens before heading back to ATL?

If you have any interest in roller coasters, there is a small family center called FunSpot just south of ATL but it has one of the best coasters in the world (at least according to my 16 year old) - ArieForce1.