Nice trail leading up into the Koolau Mountains. At the beginning, you can opt to take the upper or lower trail. I recommend taking the upper on the way out and the lower on the way back. They join back together a mile or so later as you are entering anyway, and it is easier to find the lower trail on your way out.
Various flora ranging from palms of various types sprinkled among the indgineous disidous trees. Various types of ferns if you are looking carefully, especially on the lower section, near the stream. Take your camera, there are some nice vistas into the valleys and opposing ridge line also.
The caves! Thes appear to be man-made, probably for drainage purposes. Several of them are accessible, and if you are not too tall you can crouch and walk through to the other end. They parallel the trail, so you come right back out on your way. I have accidentally flushed out a few bats a cou0ple of times, so take care, and don't let them scare the crap out of you as they take flight.
There is a little pavillion (four posts with a roof) up near the top with a picnic table and a view into the valley and the Koolau mountains beyond. Nice place to stop for a little snack, or a full blown picnic lunch.
The water is in the stream is cool, but not drinkable (unless filtered/treated), so bring a couple of liters with you. It is warm, and can be muggy under the dense forrest covered areas. Also, there is one area, where you are traversing downward along the ridge (on the way in), and it is a little shear drop. There are cables installed for handholds, and they are quite ample. It is slow going in this area, but safe.
Aloha and enjoy this wonderful hike!