Creative Writing - Start a paper with, "If I had this animal as a pet, what would I need...?", or "If I was a zoo keeper, how would I start my day....?"
Math - There are always simple math problems to be completed throughout the zoo. For instance, if you see that an certain animal weighs in at 600 pounds, and your child weighs 60 pounds, ask how many 60 pound children it would take to equal one 600 pound animal. Same with height. Or, amount of food the animal likes to eat. Or, how much an average baby weighs, etc.
Try to count how many spots and stripes you see throughout the zoo. Or estimate with word problems. An average tiger has x amount of stripes. There are 4 tigers in this exhibit. How many stripes are there?
Make charts and graph the different kinds of animals you see: Reptiles, Amphibians, Insects, Mammals, Birds, Fish, etc.
Geography - Take a blank map and indicate the different countries/continents animals come from.
Research - Before going to the zoo, find and take note of different habitats: Deserts, Jungles, Rain Forests, Swamps, etc. Upon arriving at the zoo, see if the exhibits are similar to your research. Take note of the animals that live there.
Scavenger Hunts - I found a great one posted on our zoo's website. I'm sure there are others out there.