This won't work if there's a power outage. I was going to suggest a CNN building tour, but that won't work without power either and I'm not sure that the tours operate on weekends. The Georgia Aquarium (near CNN Center) would also require power and might be mobbed with schoolkids on a weekend between Christmas and NYE. There's shopping in Buckhorn. All Chick-Fil-As should be closed for Sunday.

I'm surprised that no one has suggested staying at the airport and eating at OFS, although if you're getting a meal on the flight, my advice would probably be to hit the DL museum near the airport and then to to the F SC for F&B, showers, and the SkyDeck.