Definitely Powell's. We always stay a full day just for Powell's. Any of the McM's as mentioned before (heck, do a McM's tour - they are all slightly different!) The zoo is nice and OMSI is cool even if you don't have kids. If you are willing to leave the city, you can head down south a bit on 99W and do a winery tour through the valley! Lots tasty wineries in Yamhill County. And I highly recommend Eco-tours
if you want to do that. The fee is for the "touring" and you pay for all the wine and food yourself - so you set the prices, they provide the transportation. They also have a microbrewery tour as well as some other nice eco-friendly and fun tours.