Boy, you don't have a good choice here:
#1: All that traffic through Olympia/Tacoma and SR18. Once on I-90 not so bad.
#2: All that traffic through I-205 and Portland, then the bad directions through Tricities on 395. (Stay in left lane through Kennewick regardless of your instincts!)
#3: US12, White Pass, is a winding mountain road, 2 lane with turnouts. Not bad after Yakima.
Choose your poison. I would choose #3, especially if you are a Coloradoan and don't mind the passes.
I think you have at least a 9 hour day in store.