Tolmie State Park (five miles from exit 111, www.parks.wa.gov [1]) has 1,800 feet of waterfront on Nisqually Reach with swimming, fishing, 3.5 miles of hiking trails, and an underwater park with an artificial reef created by the intentional sinking of three wooden barges.
Open for day use only, the park also provides an outdoor shower for swimmers and divers near the lower restroom.
Links:
[1] http://www.parks.wa.gov