Difficult one as you will need to come south on I-5 through Everett, WA, then sort of skirt Seattle on I-405 hitting Bellevue traffic before getting to I-90.
You could miss some of the traffic and take US 2 east from Everett. It is a scenic two lane highway. You would then travel on east WA 28 from Wenatchee, pick up WA 28 to Quincy, then south to George on WA 281, I-90 to Vantage and take WA 243,WA 24 and WA 240 to Kennewick. Pick up I-82 to I-84.
I would not take US 395 but go east and take US 93 south.
Good roads for most part, great scenery and a lot less traffic.
