At the back of a mangrove-lined bay called Funaura-wan, a few kilometres east of Uehara, you can make out the lovely Pinaisāra-no-taki, Okinawa’s highest waterfall at 55m. When the tide is right, you can paddle a kayak across the shallow lagoon and then follow the Hinai-gawa (on the left) to the base of the falls.
The short Māre-gawa (on the right) meets a trail where it narrows. This climbs to the top of the falls, from where there are superb views down to the coast. From the river, walk inland until you come to a pumping station, then turn around and take the right fork in the path. The walk takes less than two hours, and the river is great for a cooling dip.
Unfortunately, it is difficult to find a tour company that will rent you a kayak without requiring you to join a guided tour (half-/full days cost about ¥6000/10,000).