All great suggestions on where to go and what to do.
I would also recommend Mt St Helens - Taidnapam campground is a nice place to stay and you can drive up to the overlook and see where the mountain blew out and down into Spirit Lake. The trees were blown over like matchsticks.
Olympic peninsula is another favorite - Port Townsend is a Victorian town and Fort Worden is where they filmed An Officer and a Gentleman, nice campground but does fill up fast. Fort Flagler is another campground nearby on Marrowstone Island. If both are booked, the Port Ludlow campground is also very nice. Chimacum (Finn River Cider is excellent there), Poulsbo (Sluy's Bakery is legendary), Hood Canal (Hama Hama Oyster Saloon), Port Gamble (old logging town, neat shops and haunted) - all in the same area.
Taking Hwy 20 back you could take the ferry from Port Townsend over to Whidbey Island and go that way (also a beautiful place to visit) and see Deception Pass. Anacortes is there (along with a couple of campgrounds) and you could take a ferry to the San Juan's. Winthrop is along that Hwy 20 route as someone mentioned already - also a neat town.
Have a fabulous trip and safe travels!