You could plant it up and fill it with tree frogs! I don't know what the cage set up is like but if you know anyone who knows stuff about electronics could hook up a small computer case fan on the vent to increase air flow if you had your heart set on a chameleon. A large beauty snake or king rat snake would look awesome in it and would use the space. There are some really cool tree agamid lizards that are starting to enter the hobby even if they are a bit pricey for captive bred animals at the moment. Good luck on your search.