Here is link to a website for Cascade bike club. They organize the Seattle to Portland Bike ride each year.
http://www.cascade.org/Home/ You will find the route they use and information on the route. If you search this site you will find suggested routes from Portland to the coast. A ride along Hwy 101 might be nice, its been a few years since I've been on the Hwy so I'm not sure of the condition of the road or traffic volume.