Two miles south of Des Moines on Marine View Drive South, Saltwater State Park (253/661-4956, www.parks.wa.gov [1]) is one of the most popular summer spots around. The sandy beach is very popular for swimming, scuba diving, and sea kayaking, and a two-mile path loops through the forest. Also here are picnic tables and a refreshment stand. Showers are available. Unfortunately, the park is in the flight path of Sea-Tac and planes land or take off over the park, so don't expect peace and quiet.
Links:
[1] http://www.parks.wa.gov