I note that you don't want to cross into Canada, and that's OK, because the border crossings can be time-consuming, especially getting back into USA. So let's concentrate on Washington State. Depends a lot on how far you want to go, but close by Bellingham the San Juan Islands are beautiful (setting of the book and film "Snow Falling on Cedars"). It's also well worth driving about 40 km south of Bellingham to the delightful little town of La Conner. The area is especially beautiful in tulip time. Go out onto Whidbey Island and check out Deception Pass at change of tide - amazing. If you want to go further afield, you could island hop by ferry right across to Port Townsend and visit Olympic National Park, then return to Bellingham via Tecoma and Seattle. The previous correspondent mentioned Mt Rainier and Mt St Helens - both superb, but further away. And if you want something really offbeat and spectacular, head east through North Cascade National Park to the unforgettable town of Winthrop.
A place that we should not overlook for readers of this web site, in particular, is the Boeing assembly plant at Everett, between Bellingham and Seattle. The tour is fantastic, and free.