What species of frogs are going into your viv ?
I'd say go with a Neo. compacta.They are pretty easy to keep alive and I have had imitators use them to deposit tads and had one in with my Costa Rican auratus and they all slept in it.It does get pretty big and I had two in my 20 high viv and one took up one whole side.
I'll second Mark's vote for some of the Neoregelia. Neo. Poquito Mah (sp?) is one that grows like a weed for me. It's actually a bit of a nuisance. Vriesea ampullaceae also does quite well. I would remove them from their pots and mount them bare root to the back or wherever you want them. Keep them humid and mist them once or twice a day and they should do fine.
I've seen several people recommending Tillandsia which puzzles me a little. Most Tillandsia are actually adapted to much drier and breezy conditions than typical vivaria provide. I haven't seen many people have much luck growing most Tillandsia in vivs for more than a year. They do tend to look quite good for several months to a year but fail to spread and slowly fade away. Maybe some others have the secret but I don't remember ever seeing a typically sized viv that had been going more than two years with a healthy Tillandsia in it.