I don't know a good route, so I'm just going to post a link to the Reach the Beach website. That route goes through Grand Ronde, so you could just follow that route in reverse. I've only ridden the Amity route, never the Portland route.
http://www.kintera.org/faf/home/ccp....6132&ccp=53756
http://www.goodsportpromotion.com/ev...s/portland.pdf