You can head south from Seattle to Portland, and from Portland you can head to the Coastal Highway. Stop in Cannon Beach, Oregon if you can. It's a really neat artsy kind of place, with lots of neat shops and Galleries.
Astoria, Oregon is neat as well. The streets are like San Fran with all the Hills, and there is a really nice, huge bridge there. Also it is where the movie Kindergarten Cop was set.
Have a good trip. If you end up inland, do NOT spend the night at Happy Times RV Park in Dunnigan, California. Unless you're looking for drugs.....
Cheers