I'm not from Seattle or Calif but I travel a lot. Definitely drive up to Ranier, Park at the north side called "Paradise". Walk up the trail to the snow line. It's almost the prettiest place on earth. Don't go much beyond the snow line without mountaineering skills and gear. You can get killed real fast.
Visit Carmel California. Neat place near San Jose. Californicators please correct me, Isn't it US-1 (the Pacific Coast Highway) that runs from San Fran to LA. I remember it as a slow but very scenic ride.
Visit the Sierras (Mountains) if you get the chance. As a former rock climber Yosemite is like mecca, a godlike place. There is no place on earth quite like it. Although I hear it's crowded now. Try Touloumne Meadows just past Yosemite.
Stop at Fisherman's wharf in San Francisco for seafood. I remember a chinaman's place that was quite excellent. Right on the waterfront.
A word of advice from a Californian named John Steinbeck, an author and traveler,
the more efficient your mode of transportation the less you see. Get off the Interstates and take the less travelled roads. (Emerson ?)
Sounds like a fine trip, I wish I were going along.