Unless you are a big "Lost" fan, keep driving right on past the Manoa Falls trailhead, and all the tourists, up the hill to the free(!) parking lot.
Be sure to stop in the visitors' center and drop your $5 into the donation box to support the cultivation of the native plants on the grounds.
The trails wind all around the arboretum, visiting many different micro-climates, re-planted with real native plants (not the tropical invaders you see elsewhere).
If you really want to see a waterfall, head up the valley for about a mile round trip hike to the Aihualama falls. It's not as big as Manoa Falls, but it's a bit more remote, and certainly less traveled.
BRING INSECT REPELLANT!!!