Mount Saint Helens is a few hours from Seattle, by car. The area is famous for a devastating volcanic eruption that occurred there in 1980. Its marks are quite visible: whole areas have completely lost their vegetation, other contain trees that died due to toxic fumes, and animals are scarce. I went there with Ruth (my preferred geologist and a former employee of the NASA), and we enjoyed a very pleasant hike.
If you want more information about the area, please check the guide page for Mount Saint Helens.