The south side of Mount St. Helens was not impacted nearly as much by the 1980 eruption and is best known as the access route to climb the summit, for Ape Cave [1], and for various hiking trails, including the Loowit System that circles the mountain. These sites are all accessible from Cougar.
Three miles north of Amboy on Highway 503, Mount St. Helens Volcanic Headquarters (42218 N.E. Yale Bridge Rd., 360/247-3900) is open Monday–Friday 7:30 a.m.–5 p.m. year-round.
The Forest Service’s Pine Creek Information Station (17 miles east of Cougar on Forest Rd. 90) is open daily 9 a.m.–6 p.m. from mid-June through September.
Links:
[1] http://www.moon.com/destinations/washington/south-cascades/mount-st-helens-national-volcanic-monument/south-side-a/ape-cave