We have stayed at Saltwater State Park in Des Moines a few times. No hookups, but that's OK for 3-4 days. It is close to Seattle. Last time I parked motorhome at a "park & ride" lot and took light rail to downtown Seattle.
There are not a lot of good choices close-in to Seattle. NW Trek Club had a rally at the KOA last fall. It is a little high, but they have the market. It was pretty good for a KOA from the 1970's.
Museum of Flight is in south Seattle.
Boeing factory tour, restoration center, and Paul Allen's flying museum are all in Everett, north of Seattle.
Seattle Center (Pacific Science Center, Space Needle) easily accessible by public transit.
New museum of history and industry on Lake Union sounds interesting.
Have fun in the Emerald City.