If you really really wanted a frog in your tank, a "island" could be an arrangement for an African Dwarf Frog or two. Here's how you do it:
buy some sort of turtle basking dock and put a bit of gravel and a small plant on it. make about 6-8" from the top of the tank, and put the frogs there. they would go off to swim, but come back up to rest. and you could put their food there too. they eat shrimp pellets, frozen, dried, etc bloodworms, or aquatic frog food. also they have the ability to just float at the top of the tank to rest and breathe.