Thanks to the decrepit railroad ties of a supply line to a military installation in World War II, exercise hikers can hop up more than 1,000 feet to the top of Koko Crater on Oahu's east shore. The crater, which appears as a mountain on the ascent, is across the highway from Koko Head, the onshore knob that lords over Hanaumu Bay, the island's (overly) popular snorkeling venue.
The Koko Crater trailhead is via a regional park that is just down the road in Hawaii Kai. The route is not dangerous, provided you watch your step, particularly on the way down. One short section is at a slope, with spaces between ties, so those with a healty fear of heights may wind up using all fours (like a simpering dog). From the lofty crater rim is a view into the botanical garden that is held in the crater below—and a grand panorama of Oahu and the Pacific. Maui and the Big Island will be in view on clear days.
The Koko Stairs are a training run for extreme sports althletes, although recreational hikers also show up. Bring water and avoid this one on sunny mid-days, when southern exposure is a cooker. Oahu Trailblazer has the deets (including how to use this parking lot to reach Hanauma Bay on crowded days).