Of course, the weather patterns seem to be so odd the last few years that it's anybody's guess, but September is typical one of the nicest/sunniest/driest months in the Pacific NW.

The previous suggestions are great. Any of them would be worth doing. With 10 days to go one way, I would really recommend that you do the coastal route down 101 and CA-1. With 10 days, you can veer inland to do Crater Lake and Yosemite as well.

I'm confused. Are you ending up in LA or San Francisco?