Likeke Falls is a relatively small waterfall but the hike is super easy and short. There is a small pool at the bottom and you can climb up almost to the top of the falls (sort of dangerous but fun). There is also an awesome lookout that faces Kailua.
The Makaua Falls hike is one of my favorites and, if the falls are flowing, is an incredible sight. This is only a one-mile out-and-back hike but you will be following the river and hopping rocks for a lot of it. Be very careful if it has been raining hard as the river can be intense and impassable.