The relatively flat, 11.5-mile (one-way) Waimea Canyon Trail fords the Waimea River several times. You can pick it up at the bottom of Waimea Canyon at the end of Kukui Trail, then follow it out to Waimea town, or hike upstream in reverse. An entry permit is required (available at self-service trailhead registration boxes). Bring mosquito repellent.
You might see locals carrying inner tubes on the upstream hike, so they can float home the easy way.