If you have the time, Sweet Auburn Curb Market is definitely the best bet as far as quality/variety. Close second would be Broad Street, which has Rosa's Pizza, Ali Baba, Naan Stop, and Dua Vietnamese. There's also a Rising Roll on Peachtree across the street from Starbucks where the far end of Broad Street dead ends. I'd say Broad & the Curb Market are about equal walking distance, though I'm not positive about that.
A block past Broad street at the corner of Fairlie and Poplar is Slice pizza. Decent pizza (more of a crispy crust than Rosa's). But their soups are surprisingly good, especially to be a pizza place.
At the corner of Central and Decatur, there is a trailer called Grab N Go that serves wings and hot dogs. The food is good; it was in my regular rotation at school. Although being a trailer, they have no seating.
At GSU, there is an Einstein Bagel's and a fairly decent stirfry place on the bottom floor of the University Center, and a Chick Fil A (and other places I wouldn't really recommend) on the second floor of the Student Center. (If you want specific directions, let me know. You can also get $1 popcorn at the theater there. http://www.gsu.edu/cinefest
They're showing Argo next week.)
And if you really have time, you could drive up Marietta to the West End places like 5 Seasons, or a little bit further to Carver's (which would be my choice).