I will dissent from the tree frog partisans. Leopard Frogs. They are hardy, you dont have to go Insane with habitat design to keep them happy, but you can definitely do so if you want, they are the perfect size so finding food is not a problem. The only issue is a tendency to be flighty, but even that settles down if you put them in a spot that gets moderate traffic (a well used study, for example).

I could be biased though. I keep 3 species and over a half dozen individuals.