Baltimore's zoo, while overshadowed by the aquarium, is a city charm nonetheless. This is one of the few zoos where you can find chimpanzees, and the only one to my knowledge where you can feed a giraffe. Just be sure to take the bus from the gate; it's a long walk.
The zoo features an excellent Africa section, a good arctic section, and a fun swamp to venture through.
Other that what was mentioned above, that's really all there is, with no reptile house or aviary. If you've already been to the excellent National Zoo, you won't find many additions here.