Tree frogs are nocturnal, so not very entertaining while the lights are on. You could keep 1-2 Gray Tree Frogs in there but it's very cramped for frogs that big. I would suggest you bight the bullet and get some thumbnail dart frogs. They are far easier to maintain than tree frogs once you figure them out, and they are active only during the day/light period, so you'll actually see them doing things, unlike tree frogs.