I saw you post in the Idaho forum, so you must be planning some sort of roadtrip. I suggested that you hit up North Idaho, so the best reason to go to Washington is its probably the easiest way to get to Idaho!
I think it would be a cool roadtrip to go up the coast through Oregon, which is very cool, hit up a rainforest by the washington coast, then turn east and head through north idaho. From there you can keep heading east if you're looking for a loooong trip. This way you hit the best part of Idaho (the north), the best part of Washington (the coast), all in a logical progression.