You have a few options, but try to go with a smaller species of tree frog, or a dart that likes to climb, which would land you in the thumbnail area. Not too sure thumbnails are good for beginner frog owners though, they're VERY small and quirky and will look for any escape (I've heard).
I actually have a ten gallon long that I was planning on turning into a vert to house a pair of Dendropsophus leucophyllatus, or Clown Tree Frog. Here's a link to the meet-the-frog article with some pictures. They're great looking frogs, but very fast and I've heard they're quite reclusive, so it really depends on your tastes.
Frog Forum - Meet the Frog: Clown Tree Frog - Dendropsophus leucophyllatus
But whatever you decide, just make sure you batten down the hatches tight and be sure you provide the proper temps and humidity for them. Darts require higher humidity, so I don't think keeping them outside would be the best idea...but I've never owned them, so I'm definitely not an expert.
Hope this has helped.