I really enjoyed Portland when I was there, but I wasn't on a family vacation.
I spent a day and a half there as part of another trip, and saw my favourite band and drank a lot of the local micro brews (of which there are a lot). Probably not family friendly activities.
They do have some cool things at the OMSI (Oregon Museum of Science and Industry), which is basically a really nice science centre, plus there is a submarine you can tour. Not enough to fill up a whole vacation, but it's a start.
There is also a really cool Aerospace museum not to far away (can't remember how close, but probalby close enough) where they have the Spruce Goose. So there's also that.
here it is:
http://www.evergreenmuseum.org/
Guess it's about an hour from Portland.
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