In addition to the many great suggestions already made, here are a few more:
Rent a (road) bicycle and ride a nice 2-hour loop along the waterfront (starting at OMSI and going along the Springwater Corridor)
Rent a (mountain) bike and bike in Forest Park (near NW 23rd)
Go to Council Crest Park, the highest point in Portland. Great views of the city and, if the weather is clear, at least 4 glaciated peaks.
Attend the farmer's market at Portland State University on Saturday mornings
Walk through Washington Park (head up Burnside to NW 23rd and then enter the park). From here, you can access the rose gardens, the Japanese gardens, Hoyt Arborteum, and the zoo.
Go skiing at Timberline lodge (they open for the season this weekend).
Go surfing at the coast (you can rent everything you need, including a warm wet suit, at Cannon Beach)
Go hiking on the coast: Saddle Mountain or Neahkahnie Mountain is good
Go hiking in the Gorge: numerous options of varying difficulties, consult Google
See Multnomah Falls, and take one of many numerous hikes in the area
See a movie at the Bagdad, a neat theater, and walk around the neighborhood there
Hike in Forest Park. The Wildwood Trail goes on for 30 miles. Pittock Mansion is cool. The Audobahn Society is a convenient stop.
See Mt. St. Helens. I'm not sure if the visitor center is open.
Drive to Hood River (can be combined on a long day trip to Mt Hood)
Drive to Mt Hood and hike up to the glacier
Go wine tasting in the Willamette Valley (numerous vineyards available)
Attend free art show on First Thursday
Rent a kayak and paddle the river, or go on a river cruise