I think this question would do with a bit more detail: as the Pacific Northwest is three states: Oregon, Washington and Idaho.
I live in Portland myself, and am rather partial to our city. Portland has a LOT to do, fun city history tours (used to be an old logging town), with the Oregon Coast about 2 or so hours away west and the gorgeous Columbia Gorge a nice hour's drive or so out east, where you either hit the highway and stop at Multnomah Falls, or take the old scenic highway and stop at several beautiful waterfalls including Crown Point, a great viewing spot, before rejoining the highway and heading out to Hood River, where the windsurfers and kiteboarders go to have their fun. (We know more than one Portlander who summers in Hood River for these sports.) In town, both the Chinese Scholar's Garden (a beautiful treasure of a city block) and the Japanese Gardens in Washington Park, which overlooks the city-- these are wonderful spaces to walk through and experience. LOVE them! Washington Park also has their Rose Test Gardens where you can stroll through hundreds of varieties of roses. The Oregon Zoo is close, thre's a Children's Museum next to it (if you have little ones) and our science museum is on the east side of the Portland waterfront.
Ultimately, it really depends on what you would like to do as a family. Eugene is a nice university town with a still-provincial atmosphere-- on a Sunday, many businesses are still closed. Ditto with Corvallis... both are also about 2-3 hours from the coast as well. While these are both popular university towns, they don't offer as much in the 'sightseeing realm'.
Seattle, up north on I-5, has a lot of neat attractions as well. We plan on taking our summer family trip there this year. It's a 'city' on a much more intense scale than Portland is, far more people and loads of hills to walk. If you've ever been to San Francisco, the terrain is similar, so bring comfortable shoes. There are certain swaths of the downtown area we tend to avoid (there are a larger homeless populations in both Portland and Seattle), just as we do in Portland, but also plenty of fun things to see and explore. Seattle has a well-developed waterfront area with ferry rides, curio shops, a neat aquarium, Pike Place (the complex of small shops and restaurants which overlook the waterfront) and a lot of other places we ourselves have yet to explore ( their art and interactive science museums are on our list); nearby in Tacoma their Point Defiance zoo and aquarium was a fun visit (that was in my teens, I'm in my 40s now) and I think Seattle may have its own zoo and botanical gardens.
I know that's a lot of information, it sort of depends on what your family likes to do and the ages of your kids.