The Forest Service maintains several fee campsites in the area around Trout Lake , including Tahklakh Lake ($17) and Ollalie Lake ($14).
The section of the Gifford Pinchot National Forest surrounding Trout Lake, roughly between Mt. Adams , Mount St. Helens , and Mt. Rainier , is dotted with all manner of great spots for dispersed camping. Forest Roads 23, 25, and 90 are good places to begin looking.