The Silver Lake Visitor Center
http://www.parks.wa.gov/interp/mountsthelens/
The best place to start. Good orientation exhibits, restrooms, a view of the mountain in good weather, a display showing the Volcanocam (check the view at the top before you commit to the hour's drive), and, in season, espresso.
8.5 miles from the Castle Rock interchange along WA 504
Photos by James Maynard © 2009 - free use with attribution