Sunset Crater is the companion to Wupatki National Monument as they share the same access roads. The area around Flagstaff was historically volcanically active and the San Francisco Peaks that you see dominating the horizon are volcanic in origin. To get a sense of the scale of the volcanic field here take a look at a Google satellite view of the area and the numerous craters will be apparent. For further background to the geology of the area take a look at the fact sheet here.
The monument preserves the cinder cone of an extinct volcano, Sunset Peak and the solidified lava flows that remain from the eruption. The landscape is unusual with the ground underfoot being composed of black cinders and the lava flows giving an alien feel. In fact the area was used by NASA as a training area for the moon landings as the landscape was considered to be similar to what would be encountered on the moon.
The visitor centre provides the usual amenities and has an exhibit explaining the park and the landscape. Further into the park the “Lava Flow Trail” is an easy educational 1 mile walk around a lava flow. A trail guide is available that explains various points of interest in the landscape including lava tubes and bombs.
It is no longer possible to hike to the top of Sunset Crater itself but the “Lennox Crater Trail” ascends to the top of an adjacent cinder cone. Again a round trip of around a mile but the ascent is on cinders and is reasonably strenuous.
The NPS website is located here.