Close to metro Honolulu and a destination for "adventure tour" buses, Manoa Falls is not a wilderness experience. Parking is five bucks. But get over it and you will enjoy this jaunt (1.5 miles round trip, 800 feet of elevation) through lush forest of bamboo and enormous banyans that reaches the classic tropical falls, dropping 200 feet into a pool. Trekkers can continue past Manoa Falls on the Aihualama Trail, which connects with a network of other trails of Mount Tantalus. If you're not continuing, check out Rainbow's End snackshop near the parking lot where they serve the best rootbeer float on the island.
If you go to Manoa Valley for this hike, don't miss the Lyon Arboretum, which is very close to the trailhead. Dating from 1918 (and run by the University of Hawaii since 1953), the arboretum offers garden paths as well as trails. The Aihualama Falls Trail is a floral wonder (seriously), passing towering monkeypod trees and flowering heliconias with leaves the size of surfboards. Page 78 of Oahu Trailblazer has more details.