Personally, I still think there's not enough water for the fire-bellied toads.

Darts would be cool, or some of the smaller treefrogs that Kurt mentioned. Clowns are my dream frog. It's true that treefrogs do spend quite a bit of time in the top corner, but they move around quite a bit too. White's are nocturnal, so obviously much of their time is spent sleeping during the day, but as soon as I turn off the light, they get moving. And during the day, only one of my 5 sleep in the top corner. The rest sleep in different areas.