When on the Hawaiian island of O'ahu, you just have to get outdoors. But if sunning yourself on a beach all day isn't challenging enough, you can choose from the many hiking trails across the island, which are suitable for all fitness levels – and free.

Manoa Falls Trail

This 0.8-mile hike through lush rainforest is easygoing and shaded, but often very muddy. (Flip-flops are not a good idea.) The trail begins by crossing over a footbridge and a small stream, then makes a gradual incline, ending at a viewing area set back from the base of the 100-foot (and, honestly, rather underwhelming) waterfall. More than the falls themselves, this hike is most notable for the flora you walk through: bamboo, huge banyan trees, and colorful flowers. Parking costs $5, or you can get the #5 bus from Ala Moana.

Makapu'u Point Lighthouse Trail

What would otherwise be a simple, mile-long hike to the easternmost point of O'ahu is burdened with an almost complete lack of shade along the path. Avoid the most intense heat by setting out late in the afternoon, in time to catch the sunset. As well as views of the 1909-built lighthouse that sits on a 600-foot sea cliff overlooking Makapu'u Beach, you might be able to spot humpback whales during their annual migration season (December to May).

Lanikai Pillboxes

The Lanikai Pillbox Trail (also known as the Ka’iwa Ridge Trail) traces a ridge and passes two old World War II bunkers. It is a fairly easy 30-minute hike, but is almost entirely exposed and begins with a very steep climb up a dirt hill. Once you're past that part, it's a relaxed walk up to the top for sweeping views of Kailua, Waimanalo, and, on a clear day, Moloka'i. The best part is, though, that when you are done with your hike, the inviting sands and warm water of Lanikai Beach below await.

Koko Crater Railway Trail

This challenging trail is made up of 1,100 steep steps up a disused railway line running to the top of a volcanic crater. The hike begins gradually, but once you make it across a rather unnerving suspension bridge, it's a precipitous, leg-shaking climb to the top. Making the hike even more demanding is the fact that there is no shade along the way. Visiting in the morning or late afternoon makes it slightly more comfortable. Take a break if you need to (you'll often see people take a time out along the side of the railway line) but don't give up: the views from the top are incredible.

Haiku Stairs (Stairway to Heaven)

The Haiku staircase, more commonly known as the Stairway to Heaven, is a steel, 3,922-step staircase that ascends a ridge up from the Valley of Haiku to the crest of the Ko'olau Mountains. The stairs, which were originally made out of wood, were installed during World War II so that military personnel could access a radio station antennae 2,000 feet up the mountain. The stairs have been off limits since the 1980s and, for most of the day, a security guard is posted at the entrance to make sure no one attempts to climb them. Adventurous hikers have gotten around this for years by getting up at the crack of dawn and sneaking onto the staircase before the guards begin their shifts. Despite the perceived danger, few accidents have occurred along the trail (although climbing a vertical ladder up a steep mountain face is not for the faint of heart). Still, if you do decide to venture onto this one, bear in mind that it is still illegal and that doing so is at your own risk.