I'm supprised that no one here has mentioned anything about frogs, I used to keep Red Eye Tree frogs, They were awsome, you just have to make sure that you have plenty of land and plants in your tank and an easy way for them to get out if they fall in the water. Tree frogs come in lots of different sizes too, so you can keep them with some of the larger (but not agressive) fish, they do great in a tank on their own too.