The problem with tree frogs is that they are primarily nocturnal, which is why many times you see dart frogs in display tanks, because they are diurnal. However, if that doesn't bother your, then a Central or South American species would be great. Species of Agalychnis (the red-eyed tree frog and its relatives), Dendrosophus (the clown tree frogs), and Hyla (almost all the other ones), can be great. They are relatively small and gentle, so they won't destroy your carefully planted tank. Species like the White's Tree Frog (Litoria) are common frogs to keep, but they tend to be destructive of nicely planted terrariums because they are so big. You could also think about Phyllomedusa (the waxy monkey frogs), they tend to be diurnal and some of the species are more gregarious like the White's. An alternative could be day geckos (dare I say that on frogforum?). They are diurnal and often very colorful. You often see these kept along with frogs because they are also insectivorous and often pose little danger to the frog.

I hope that helps a little, and if you have any more questions, just ask.

Best,
Alex