Bill,
From downtown Portland you'll definitely have to check out the
Springwater Corridor trail, which you can get to from the Eastbank
Esplanade trail. You can get on the Esplanade from almost any of the
bridges near downtown and just head south and follow the signs on the
Springwater trail. It follows the Willamette and takes you through
some beautiful wetlands and into a little town called Sellwood where
you can grab a pastry or coffee and head back into the city!
Have fun on your trip.
--Jonathan
http://bikeportland.org