Located southwest of Coupeville [1] on Admiralty Inlet, Fort Ebey State Park (360/678-4636, www.parks.wa.gov [2]) has campsites (standard $17, RV hookups $23), a large picnic area, three miles of beach, and two miles of hiking trails within its 644 acres. The fort was constructed during World War II, though its gun batteries were never needed.
The concrete platforms remain, along with cavernous bunkers, but the big guns have long since been removed. Due to its location in the Olympic rain shadow, the park is one of the few places in western Washington where cactus can be found; it also has stands of second-growth forests and great views across the Strait of Juan de Fuca [3].
Links:
[1] http://www.moon.com/destinations/washington/north-puget-sound/whidbey-island/coupeville
[2] http://www.parks.wa.gov
[3] http://www.moon.com/destinations/washington/olympic-peninsula-and-the-coast/port-angeles-sol-duc/strait-juan-de-fuca