My favorite Zoo in the whole World is Lincoln Park Zoo in Chicago, and it's free.
I spend the morning in the conservatory and the afternoon walking around the zoo's many buildings. There are always many images to choose from: flowers, fish and workers to name a few.
In the winter not many of the animals are let out on the grounds.
So, I resort to the buildings and use flash photography to capture the animals. This will some take some practice and notes to use flash with RAW images.
If you are not shooting RAW you should for a lot of reasons. My favor reason is that one of my images was not purchased because I did not have the RAW file of the image. That's learning my lesson the hard way.