Oahu has many great hikes, a favorite being the Manoa Waterfall which takes about 1 hour round trip. Trek though dense tropical jungles (think “Jurassic Park”) and bamboo forests. At the end of the trail is a tall waterfall. A nature and bird lovers’ delight. Tips: Avoid this trail if it’s been raining, and remember to take bug spray.
Of course, the Hawaii State Parks Oahu hiking guide is a must hit when you are thinking of hiking the state’s maintained trails. On Oahu there are 8 such trails that are marked out. At the site each trail is explained, noting specific features and includes maintenance schedules by the state. Hiking permits are not required, but there is no camping. The only trail that you need to pay for is the Diamond Head Summit Trail: entry fee is $5 per vehicle or $1 per walk-in.
It is best not to just go venturing out into the wilderness of Oahu without knowledge of where you’re going. One of the best resources on the web is at at EveryTrail.com. Their Hiking on Oahu page is one of the most comprehensive, listing several popular trails with difficulty, length and ratings. If you want a more local point of view, check out UnrealHawaii.com and their Best Hikes on Oahu page where they have spectacular shots of the hike with good info and tips.
Please remember to be safe. Carry a charged cell phone with you in case you need to call for help, and always have plenty of water to stay hydrated. Some of the hikes on the island are very challenging and people frequently get caught in the high mountains due to quick changing weather conditions.