Sunset Crater Volcano National Monument is located 12 miles north of Flagstaff. Roughly 900 years ago, the volcano erupted, forever changing the lives of people, plants, and animals in the area.
Our family "hiked" the Lava Flow Trail in February 2013, marveling at the cinders and the blackened mountainscapes. This is an easy self-guided loop trail that is one-mile roundtrip. 1/4 mile of the loop is fully paved and the rest is made up of a series of natural stairs and rock paths.
Though you certainly won't want to plan a trip around Sunset Crater (especially if you have young children who may find it tiresome), the cindered mountainscape is fascinating to see. It's a worth a trip if you're passing through Flagstaff.
The entrance fee is $10 for 2 adults (kids are always free) or free if you have the America the Beautiful Pass*.
* That pass is outstanding, by the way – $80 for your whole family to visit any National Park or Monument in the USA for one full year!