One of the shortest and usually easiest hikes on Oahu is on the Manoa Falls Trail. The trailhead is easy to find, with the Rainbow's End Snack Shop nearby.
The trail can be a little treacherous in wet weather. The muddy path becomes quite slick. A good pair of shoes goes a long way. Seeing the waterfall after a good rain is the best time to visit, as the falls will be gushing rather than trickling, but Manoa rarely dries up—making it a great waterfall to hike to any time.
The Manoa Valley is also known as Rainbow Valley, so be on the lookout for some color in the sky as you drive in to the trail!