Coming down from Seattle, you have access to a lot of national parks, some of which have been mentioned. There is no shortage of scenery between Washington, Oregon and California. Your biggest challenge will be what to see (and what you won't have time for).

There's Crater Lake National Park in Oregon:



The drive over Mount Shasta into the Whiskeytown National Recreation Area:



Lassen Volcanic National Park east of Redding/Red Bluff:



And a little out of the way you have Lake Tahoe which is surrounded by national forest.



I'd do the whole west coast with a Mustang if you can swing it and deal with any drop-off charges which could make the decision for you.