Oh boy! Well, what do you want to do? Do you like outdoorsy stuff? If you do, there's a book called "Wild in the City" that has all sorts of day hikes & outdoorsy things you can do as day trips/walks/adventures. It's a great book that is full of a love for the Portland area.
But it really depends on what you want to do - you can do a day trip to the coast. Seaside is a good spot, as is Pacific City. And, if you're lacto-veg, you could stop at the Tillamook cheese factory -- and see cows happily grazing (though I'm not sure whether they use rBST).
Or take a trip up the Columbia gorge. If you haven't done the scenic highway you MUST do it - take I-84 to exit 17, take a right off the exit, go over the freeway, and make a left at the Plaid Pantry. That's the columbia highway. Follow it along the gorge and make stops at the scenic points - lovely viewpoints and waterfalls. That's the area I grew up in, and I know and love it well.
Go a bit further up the gorge and cross the Bridge of the Gods and read about the native american legends about Mt. Hood (Wy'east) and the bridge.
Or go to Silver Falls. There's a 5-mile trail loop in the state park, which is about 75 minutes SE of portland, and makes for a great day hike - and you see about 7 big waterfalls on the hike.
Or go north to Mt St Helens.
Or go to Ashland & marvel at the mouth of the great Columbia.
Or to Eugene & make a surprise visit to ebola.
If you're into ghost towns, Portland has tons of them on the eastern side of the cascades (like Shaniko). Or cross the river into Washington and go to Maryhill mansion
and Stonehenge replica. Or go back to the Oregon side near The Dalles and see the "She Who Watches" native american pictograph.
I love oregon.
I hated that my dad made me do all that daytrip stuff when I was growing up, but now I'm so glad he did it.