Powell's (as mentioned)
Hawthorne Street *or* 23rd Ave NW. Pick one if it's a short trip. Hawthorne is less walk-y fun, but has better shopping deals. 23rd is gorgeous, but is pricy. There are good pubs and clothing stores on both (including a Buffalo Exchange on Hawthorne in a town where everyone needs to sell and no one can buy...stock up), but definitely make time for The Mission (NW) or The Bagdad. They are the movie theaters owned by McMenamin's that serve food and booze during the movie. You may say we have Central Cinema, but it has NOTHING on these.
If you are going when it is nice out, hit the Pied Cow. It's in a Victorian house with the side yard converted for outdoor seating. On a sunny day it is what I would choose for my afterlife.
For actual touristy stuff the zoo there beats the pants off ours.
If you can do a Saturday, the Saturday Market and river front / Pearl District is a very nice day. Portland's Chinatown has been shrinking, so you can fit that in this day too.
If you want the best hair cut of your life, make an appointment with Morgan at the NE Davis Bishop's. Srsly.
OMSI, the Japanese Garden (attached to the zoo), the rose garden, Pittock Mansion and the classic Chinese garden in Chinatown are also fun.