I don't think that is enough time to do the Aquarium justice. How long is the tour for the new Coke museum? I haven't toured the new coke museum, but that admission has like quintupled since the last time I went so hopefully there is a lot more to see too.
All around that area is the Olympic Park, CNN, Coke, Aquarium. Maybe just walk around and explore that area (or head over to Underground).
You probably don't have time to make it to Stone Mountain but that is the touristy place in ATL that is on the top of my "attractions to see again."
Keep your money close to you as you will unfortunately probably see many homeless people on the MARTA or if you walk from CNN to Underground.