I had a pet Green tree frog a long time ago, and didn't find him boring at all. Like all tree frogs (as far as I know) he was nocturnal, but he wasn't completely inactive during the day, I don't think (it was a very long time ago).

I personally think that a 12x12x18 is probably a little small for a Green. I kept mine in a 20 gallon tank, and that's about as small as I'd go with this species.

If you want active during the day, you pretty much have to go with dart frogs or mantellas (or a few that are even more expensive).

For what your saying that you want, and with the available space, I would consider going with a pair of thumbnail dart frogs (there are a few types that are relatively inexpensive). Another possibility might be getting a couple of Bumblebee Toads. They have CB ones, relatively inexpensive, at Josh's Frogs right now, and their space requirements are very modest. I know Green tree frogs are cheaper, but it doesn't sound like they are really what you would want.

Good luck, and hope you can find something that you will enjoy keeping.