Mt St Helens is a good scenic road that i can think of, thats between seattle and portland. Might be longer for seattle to cruise to Mt. St. Helens, but well worth the drive. And the mountain is a great background for a photoshoot.
Another would be Mt. Rainier or Packwood to the south of Mt. Rainier. If we wanted to, we could meet up in Centralia and take the back roads through Tenino and up to Morton. But thats the only route I could think of that would on top of my head.
I think skid only knows roads that are up north. Chuckanut is way up north by bellingham. Dont think it would be worth the drive, unless we plan to cruise into BC. Now that would be an awesome cruise. We would, however, need to meet up at 5am lol