I will "third" the Banks-vernonia. Nice ride plus a scenic wooden trestle - and those are getting rare. I try to stop in each year on my way north in spring. There is a new state park and campground (s) in the middle of the trail - Stub Stewart.
Go to Traillink.com and check out the offerings. Here is a link to Portland. There is a lot of nice riding there.
<http://www.traillink.com/trailsearch.aspx?city=portland&state=OR>
For a quickie tour of the river front on both sides in downtown, take a look at the Vera Katz. This can be linked to the Springwater on the Wilamette, which takes you to the Springwater Corridor
<http://www.traillink.com/trail/vera-katz-eastbank-esplanade.aspx?gce=201304_2&utm_expid=5284793-5&utm_referrer=http%3A%2F%2Fwww.traillink.com%2Ftrailsearch.aspx%3Fcity%3Dportland%26state%3DOR>
Enjoy - and remember it does rain out here in the NorthWet.