I've done the Willamette Valley ride several times now. I always vary the route somewhat. It is quite beautiful wandering through farmlands. There is always a good chunk of riding from Portland to Champoeg, but I've been experimenting with new routes for that section.

I haven't been on the Banks Vernonia trail, but that could make for a nice day trip.

Banks-Vernonia State Trail - Oregon State Parks and Recreation

You could probably also choose a route to ride to Astoria.