Native American Cultures. PNW Indians are not what you see in Westerns. Check them out.
Fascinating indeed. Our native tribes were much different than the well-known, nomadic, tepee-dwelling natives from the plains and midwest. PNW tribes built permanent homes and civilizations, and thrived on a marine-based economy and lifestyle. And since this part of the country was largely unexplored as recently as 150 years ago, much of the natives' history and art has been well-preserved and documented.
philly cheese
Olympics. No, not the competition but the isolated mountain chain rising west of the Sound and providing some great wilderness.
The Olympic Peninsula is grossly under-rated. It has to be one of the most naturally-beautiful places on this earth. And it's still largely unpopulated, making for some very pleasant, scenic driving as well

. The Hoh Rain Forest is there (pictures do not do it any justice), as well as the Pacific Ocean. And of course, the mountains are there; very picturesque. Evidence of the Olympic rainshadow is also a bizzare trip. The wettest city in the state (Forks, 120 inches of rain per year), is 60 miles from one of the driest. Sequim gets 8 or 9 inches of rain per year, and looks like a desert...sandwiched between the mountains and the Strait of Juan de Fuca.
philly cheese
Eastern Washington is pretty lacking.
The Columbia River and Gorge is worth a peek, as well as the transition between West and East. Literally 20 minutes after you go through Snoqualmie Pass on I-90, the trees, grass, and lakes disappear, and you're speeding across a flat, arid plain, wondering how it is that 4 million people live only an hour away.