Vancouver could get complicated
You will need to check to ensure that you can enter Canada and return to the US on your current traveler's visa. I don't pretend to understand the rules but you definitely want to check this out so you don't get stuck in Canada. Also, you will need to check with your car rental agency to ensure that it's OK to take your car into Canada.
If you can get to Vancouver, it's lovely and you could always do a quick stop in Seattle as well. Both cities have a lot to offer.
When in Washington, I always suggest that people stop at Mt. St. Helens. The blast zone is still incredible to see.
Check out the links in this post for ideas. I'd definitely travel the Oregon/Northern CA coast on the way back to San Francisco. It's the way I'd go. Beautiful with lots to see and do. Peter's roadtrip report should also be quite helpful.