You might go north out of Seattle and take the North Cascade Highway(highway SR 20 I think). Incredible country, incredible drive through the mountains. then make your way south through the aforementioned farm country. If time allows you could travel across to Spokane and down through the Palouse to Lewiston-Clarkston and take a jet boat trip up Hells Canyon You won't be disappointed. From Clarkston travel to the Walowa's (the Oregon alps) cross Oregon to Redmond area through the cascades and up the Oregon Coast (spectacular coastline). Cross back over the coast range into the Willamette valley (farm country) Up I 5 back to Seattle ...........whew...............what a trip.............could from Portland sneak up the freeway to Multnomah Falls for a picture or two....................

There is allot to see in the northwest, 10 days, and allot of miles, and you can see allot. But you will miss allot as well......

From Spokane you could venture over into Idaho, Coeur D' Alene area then down 95 (past my place, even could through a burger on the barby ........) on your way to Lewiston.

The above may be a bit ambitious 1500 miles easy, but many variations could be made that would be very interesting. I guess it depends on how you like to spend you time. Break your trip down to a couple hundred miles a day and you can see allot of country.