I like Multnomah Blvd. but they've been doing contruction on the I5 end which will detour you. I recommend the spring garden/Taylor's Ferry route up to Terwilliger (S). From there take the residential route suggested by Google along Lobelia-5th-Primrose-Palatine Hill-Riverside (at the collage). Hall and Olsen have decent enough bike lanes. You will be doing some climbing but nothing impossible; relatively gentle.
Check it out on MapMyRide: http://www.mapmyride.com/routes/view/59321348/