Downtown Atlanta is a little restaurant unfriendly, but there are a couple of good places within quick walking distance of the Marriott. I would recommend Googie Burger, which is in Centennial Olympic Park right next to the fountains, where you can enjoy a very good burger and some of the best milkshakes around. There's also Pittypat's Porch, whose glory days are long gone, but you can still get a pretty good southern dinner there. The problem is that the downtown is really dominated by the Americasmart convention space, which covers several city blocks and prevents any real interesting development around those hotels. As you walk from your hotel to the aquarium, you will swear you've passed the same building already.
You can't go wrong with any of the Atlanta recs here in the Restaurants section - Mary Mac's, Silver Skillet, Varsity, Fat Matt's and Manuel's Tavern are all very good. Harold's BBQ is awesome, but I wouldn't want to be standing outside there waiting for a bus after supper.
Our public transportation is a long way behind Boston, Chicago or NYC, but it gets the job done. It's called MARTA and you can find the bus you need at http://www.itsmarta.com
The Varsity and Mary Mac's are both within walking distance of the North Avenue train stop, but I'd plan to get a cab back from Mary Mac's. You'll be too full to walk (uphill) after a supper there.
Some other current faves in the city include Delia's Chicken Sausage Stand (chicken "slinger" hot dogs in a business fronted by the lady who started our popular Flying Biscuit restaurants) and Grindhouse Killer Burgers (small but very tasty patties, sort of the local opposition to the Smashburger stores that are starting to open in town this month).
Flip Burger is quite good, too. As Nancypalooza notes, it's on the same road as Bocado and Leroy's Fried Chicken and Yeah!Burger and Figo Pasta and West Egg and all the other current faves. You could probably hit 'em all on one bus route if you really wanted and your credibility with Atlanta foodies would be assured.