I volunteered at the Trevor Zoo (aka Millbrook School Zoo) in NY. I was there for ~ 5 years as senior volunteer, but they had a period of poor leadership during which I felt the animals and facilities were not being properly cared for, and I quit. They have since changed employees and the place is now back to its beautiful, well-cared for self, but I don't really intend on going back since I've been a little dissillusioned by thier previous lapse.
At any rate, with the exception of the conflict that caused me to quit, I think it was a fantastic experience. It was a pretty small zoo, but AZA approved with gorgeous exhibits and SSP work. My work there included twice-daily feedings, exhibit cleaning and maintenance (ie. weeding, fixing fences, etc), building cleaning and maintenance (sweeping, mopping, emptying drains), behavioral enrichment duties, composting, and caring for the mice, rats, and gerbils in the live feed room. I also got plenty of exposure to the wildlife rehab program, which is what encouraged me to get my own liscence, and even got to adopt a few exotic pets who had been abandoned at the zoo's main building (something which happened with truly depressing frequency).
If you can get an opprotunity to volunteer, I strongly suggest it. It is wonderful experience, will look great on a college app, and will strengthen your understanding of animal behavior.