The Kilauea Iki Trail in Hawaii Volcanoes National Park is one of the most popular in the park, but I’ve never found it crowded when I’ve been there. It features two very different environments, the walk around the heavily wooded crater rim and the crossing of the barren lava on the crater floor, about 400 feet below.
Since the trail is a loop, one has to descend the steep crater wall at one end and climb up the equally steep crater wall at the other. At both ends, the trail is heavily switchbacked to make this possible. For the hiker, the positives of this are that the trail is less steep than a straight shot would be, and the switchbacks are good spots to take a break and get a variety of views.