Halona Blowhole is an awesome sight to behold on the windward side of the island—you may have to be patient but the water can shoot up to 20 feet into the air!
The actual trail for this hike is one of the best/easiest, flattest, and best maintained trails on the island. The hike is pretty easy but there are some strenuous inclines as these falls are located near the top of the mountain. Along the way is a fun swimming spot with a slide and beautiful valley views. The trail is very pretty, usually with thick foliage on both sides.
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.