Marckworth Forest offers some wonderful walking among forest, marshes, and rumpled rocky hills, as long as you're ok doing your own routefinding on logging roads. It's a quiet, little known and infrequently visited spot, less than an hour's drive from Seattle.
Marckworth Forest is made up of state trust lands managed by the Department of Natural Resources as a working forest. Timber harvesting here helps provide revenue to beneficiaries like schools and universities. While Marckworth Forest is open to the public for dispersed recreation, it does not have designated recreational trails or facilities like trailheads, designated parking areas, or campgrounds. Visitors may encounter active timber operations and logging trucks, or other vehicles, on forest roads and should be alert when recreating here.
That also means you'll want to come prepared. While there are plenty of routes to explore here (mostly on logging roads), there are no signs or maps to guide your way. Dispersed recreation is permitted, but because there's no formalized trail system, your experience here will likely feel more remote and rugged than many trails in the area.
Many folks explore the Stossel Creek area, which is where the driving directions below guide you. But there is no designated parking area. If you go, be sure to park in such a way that respects any posted signage, and traffic can flow unobstructed.