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 Tunnel Fire in 2022 heavily damaged the Lava Flow Trail---a half-hour, mile-long, self-guided walk that provides a good view of the lava formations and holes in the rock where volcanic gases vented to the surface---which is now closed indefinitely. The fire also damaged and closed the Lennox Crater Trail, which climbs 280 feet to the top of Lenox Crater; should it reopen, wear sturdy, closed-toe shoes since the cinder there is soft and crumbly. Lava Flow, Bonito Vista, and A'a trails are still open despite the fire.
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 2.5-mile hike to the top. 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.