I've kept terraria in the past, and the best plants are those that have don't need lots of air circulation and don't mind being fairly damp all the time. In the frog tank they'll also need to be non toxic. Some of the best would be ferns such as rabbits foot fern, bird's nest fern and small Boston ferns or maybe button ferns - most of these you can get really cheaply as baby house plants at the nursery (or even supermarket). You could also try baby's tears - very small leaved creeper that looks a bit like moss. There are also things like philodendrons which will climb and can be trained. Spider plants are also very easy but do need a bit of light. If you have really low light peace lilys will be fine.
Anyway - those are the ones I'm looking at to use!