How far are you willing to drive to hike?
The Columbia Gorge is within easy driving distance of Portland and has lots of hikes with beautiful scenery. We're thinking of doing a trip this summer doing a loop up past Punch Bowl Falls.
Table Mountain in the Coast Range is about 45 minutes out of Portland, and while I've never done the hike myself, I hear it's great.
Mt. St. Helens is also within relatively easy distance of Portland--about an hour--and has loads of hiking opportunities. I really really recommend Mt. St. Helens if you've never seen it before--how nature has recovered in the area is fascinating, and the scenery can't be beat.
Tell me what kind of scenery you want to see/how far you're willing to go, and I can point you in the right direction. I'm not at home at the moment but I have several hiking books for Oregon/SW Washington.