Sunset Crater, a cinder cone that rises 1,000 feet, was an active volcano 900 years ago. Its final eruption contained iron and sulfur, which give the rim of the crater its glow and thus its name. You can walk around the base, but you can't descend into the huge, fragile cone. The Lava Flow Trail, a half-hour, mile-long, self-guided walk, provides a good view of the evidence of the volcano's fiery power: lava formations and holes in the rock where volcanic gases vented to the surface.
If you're interested in hiking a volcano, head to Lenox Crater, about 1 mile east of the visitor center, and climb the 280 feet to the top of the cinder cone. The cinder is soft and crumbly, so wear sturdy, closed-toe shoes. From O'Leary Peak, a 5-mile hike from the visitor center on Forest Route 545A, enjoy great views of the San Francisco Peaks, the Painted Desert, and beyond. The trail is an unpaved, rutted road (closed during winter), with a steep 2½-mile hike to the top. Bonito Campground, just outside the monument, is open from May to October ($18) and has toilets but no hookups. To get to the area from Flagstaff, take Santa Fe Avenue east to U.S. 89, and head north for 12 miles; turn right onto the road marked Sunset Crater and go another 2 miles to the visitor center.