I vote for dart frog!! A properly done set up can look amazing. They do have strict requirments, but if you check into it and fell comfortable, they get my vote!!!
I'd put a bearded dragon in there, but that's just what I want to put in my big (not sure how big it is yet, I haven't quite gotten it home from the friend who gave it to me). Or maybe a blue-tongued skink? They look kind of cool. Could try some Uros, ackies or a snake of some sort. Just look up some different species and see which one you could care for best.