Have you identified the moss attached to a couple of those pieces?
Depending on humidity level, and if you do wind up trickling water down the larger piece, you might look into adorning the section above the water line with some smaller orchids, tillandsia ("air plants"), and other humidity-loving epiphytes. Perhaps draped with some longer, hanging mosses? You can often easily find some varieties in the indoor section of local greenhouses. Could be very beautiful!