Maunawili Falls, Oahu
Oahu is not known for its waterfalls, but that doesn't mean that the there aren't any. Maunawili Falls is an excellent hiking trail that leads to a small waterfall and pool that you can actually swim in. Maunawili Falls is located in Kailua on Kelewina Road. Good hiking shoes are recommended, as it does involve crossing three streams that can be slippery. In addition, it is not recommended that hikers cross the streams after heavy rainfall. During the hike you'll see banana palms, wild ginger, heleconia and even wild coffee trees. The hike also includes some spectacular views of the Ko'olau mountain range. Remember to pack a bathing suit if you hope to swim at the falls, but don't drink the water, and don't swim at the falls if you have any open cuts due to leptospirosis.
Ho'opii Falls, Kauai
The island of Kauai is famous for its numerous waterfalls, but many of the waterfalls are hard to get to. Ho'opi'i Falls is one of the few waterfalls that you can actually hike to. There are actually two falls--an upper falls and a lower falls. To get to the falls, park at Kapahi Road and follow the dirt road until you get to the trail. You will want to follow the path downstream until you get to a side trail that leads to the upper falls. A nearby lava bench allows you to sit and watch the cascading waterfalls. To get to the lower falls, continue another half mile down the trail. Be aware that it might seem like you need to cross the stream, but you actually don't--and shouldn't, as it will take you to the edge of the falls. Instead, look for a spur trail that will lead you to the bottom of the falls.
Pipiwai Trail, Maui (picture above)
The Pipiwai trail is located along the road to Hana. The trail can take two to five hours, and will not only take you to the base of a 200-foot-tall waterfall, but to another even larger waterfall beyond called Waikamoku Falls. The trail is located at the 42-mile marker on Hana Highway. Less than a mile into your hike, you will get to the base of Makahiku Falls. Continue on the trail through a large bamboo forest until you get to the even more impressive 400-foot-high Waikamoku Falls.