I'm on Oahu, actually lived right beneath the Koolau mountains in Kaneohe for most of my life! Not sure when you/your wife was last here... but the relatively new H-3 freeway has the best views of the mountains. Tons of fern with twisted O'hia trees and silvery Kukui nut trees poking out. Stunning, really.
Fissidens would totally rock covering the wood!
O'hia would make very nice driftwood, but an invasive species, that **** guava tree, old creek side pieces can be found that hollow out and look good, mostly on the north side up some of the canyons with streams, there's bound to be plenty. These are very suitable for aquariums.
Super bad for the ecosystems, good for planted tanks. Rock? Well you have one choice really there

We where there mid Jan this year, her folks live in Wahiawā. We go most every year and then go to a different island each time after visiting, Kauai this year.
Regards,
Tom Barr