If you are going to the coast, you migh check out the Tillamook Air Museum.
Tillamook Air Museum I like it because it has prop planes from the past and you can get close. Not a super polished, high brow experience, but very cool. I also hear that there is a autos and airplane museum up the Columbia Gorge in Hood River that is great.
I think the first question is, when are you coming and other than airplane museums, what do you like? Portland and beer is a no-brainer.
Larry