It's been a while since I've been to the zoo, but I think there is at least one sit-down fastfood place at the zoo itself, in addition to lots of snack bars and roach coaches. It's quite a walk from the zoo up to Conneticut (?) Ave, and I believe it's mainly in a residential area. Having said that, I live out in rural northern Virginia, so someone who lives in the District or closer might have better ideas.