The Forest Service’s Lower Falls Campground (15 miles east of the Pine Creek Information Station on Forest Road 90, 518/885-3639 or 877/444-6777, www.recreation.gov [1], mid-May–mid-Oct., $15 tents, $35 RVs, $5 extra for reservations) is a fine camping place, with views of three large falls along the Lewis River and a hiking trail that heads downriver for 10 miles.
Several other non–Forest Service public campgrounds provide camping on the south side of Mount St. Helens. Near the town of Cougar, Cougar Campground and Beaver Bay Campground offer tent camping ($17) and showers on the Yale Reservoir along Forest Road 90. Go north on Road 8100 to Merrill Lake Campground, a free Department of Natural Resources camping area, or head east on Road 90 to Swift Campground (just south of the Pine Creek Information Station, $17). Reserve a spot through Pacific Power and Light (503/464-5035).
They also have very popular summer-only campgrounds at Cresap Bay Park (seven miles north of Amboy on Hwy. 503, $17) and Saddle Dam Park (near the Yale Lake dam on Frasier Rd., $17). Cresap Bay and Swift Campgrounds have campfire programs on Friday and Saturday nights June–August.
Links:
[1] http://www.recreation.gov