Swift Forest is a huge 92 site campground at the Junction of Mt St Helens and Gifford Pinchot Forest. We thought we might have luck here as a Father's Day weekend walk in since the campground is so huge, but it was completely booked on Saturday. Went back on Sunday and the campground was nearly empty by afternoon. 

Sites here are $21 with an extra $5 fee per night for booking online (ouch).

The gated campground is very clean and the hosts are very attentive. No alcohol.

There is a large covered picnic area near the lake, a marked swimming area (f you like freezing cold water) and a boat ramp. Life jackets are available.