I am not aware of any ferns that are toxic to frogs. Just remember that down the road, all ferns grow big. Expect it to die off the first few weeks, then you'll start to see new fronds appear. Most begonias will be fine. They tolerate humidity rather well. Some do get large, so research the species before you buy. Violets would also work, but they really don't like staying wet, so if you go with violets, they need to be in a dryer section of the enclosure, if there is one. Episcias (?) Are also good, as well as jewel orchids