Not a scenic route per se. This is the shortest route I could devise. It avoids major highways when there is minimal difference in distance, or its illegal to ride the shoulder (around the cities).

Chuckanut Dr. through Larrabee State Park is gorgeous, with spots for taking a scenic lunch.

Everett is the most unpleasent leg of the trip. Hills and lots of thick stop and go traffic through strip mall territory.

The side roads from Longview to PDX are quite hilly, I-5 is very flat however, so if you want speed and don't mind the interstate, take I-5.  Around Salmon creek you'll have to get off the interstate though.