Pololu Valley cuts into Kohala mountain and offers some fantastic views of the coast. The best viewing areas can be found part-way down the trail heading down to the valley floor, and the whole hike down is less than half a mile, making it much more accessible than Waipi'o Valley.
Once you get to the bottom there's a black sand beach and some trails leading around the valley. There's also a trail going up the other side through some dense forest and fruit trees, though when we took this we ran into a gate at one point and didn't see the views we had heard about. Maybe we missed something.
I wouldn't recommend going into the water, as the current is fairly strong here.