Make sure to spend the whole day here. It's most notorious for its major 1980 eruption, the deadliest and most economically destructive volcanic event in the history of the United States.
Long drive but well worthwhile. Nice visitor center with very nice staff. Better if you can do the hikes to the cliff dwellings. Come early if doing the hikes.
This was a fascinating place to go to. If heading from North Rim of the Grand Canyon or the Southern UT area to St. George, definitely take some time to stop in here.