Mount St. Helens is a must see. Sitting atop Johnston Ridge where you can gaze into the crater and read the scars of the landscape provides one of those rare encounters with the fragility of life.
Trivia bits that were difficult or impossible to pick out of the usual tourist presentations: - The small craters in the plane below the north face were caused by the pyroclastic flow blanketing the mudflow of the initial landslide. Pockets of steam were trapped and burst through the pumice as small geysers. - It is mentioned a few times that those within 50-60 miles of the blast did not hear the eruption, but only the secondary destruction of the forest (moving about 150 mph towards them). Apparently, the initial shock wave (post-landslide) was focused upwards, and the only reason those 50-200 miles away heard it was a reflection off the stratosphere.