The one exciting hike I took was the walk from Waipio Valley over the top of the pali to the next valley, Waimanu. Waimanu was almost deserted then, although there were a couple of people living on the beach. The trail is about nine miles one way from the bottom of the Lookout road. It goes up and down over the pali a dozen times before you will finally descend into Waimanu Valley through a forest of lauhala trees. Many of the times we descended, we were greeted by a waterfall, often with a deliciously cool pool below it. You can make it into Waimanu Valley in one day, but there was also a shelter about two-thirds of the way along when I hiked the trail. I understand that large portions of the trail were destroyed in floods since I hiked it but have since been rebuilt by volunteers. If you are interested in camping in Waimanu, you can get more up-to-date information and make a reservation at the State's site.
The Big Island offers some of the most challenging hikes in the islands, including a trek to the top of Mauna Loa, almost 14,000 feet in the air. The Mauna Loa Trail offers a multi-day hike and is not for the occasional hiker. Complete lightweight hiking gear is required and the hiker may encounter snow, rain, extreme cold, high winds and, even, fog, but for the real adventurer, this could be perfect trek. You must register with Hawaii Volcanoes National Park headquarters for this hike. For more information on the Mauna Loa hike and other hikes, please visit the park's hiking page.