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Museum of Flight, straight up.
Also Living Computer Museum if you’re into retro computing.
Got money? Get a seaplane up to the San Juan Islands. Stay in a B&B, eat good food, chill. Orcas Island is nice. Hike Mt Constitution or whatever.
Definitely the “thing” to do would be to drive a circuit around the Olympic Peninsula, taking the ferry over the Sound. Can’t go wrong with all that. Just a bit busy if you hike the popular trails. Lake Quinault is ace, so is Lake Crescent. The coast is meh, beaches are busy and cold. Others provided good suggestions for the peninsula.
Seattle has a pretty good fine dining scene if you’re into that (make reservations now). Hotel Ballard is a good place to stay, and if so, Nordic Museum nearby there maybe worth a quick visit, aside from regular libshit stuff.
Seattle is a write off. Don’t bother with the regular places; Space Needle, Starbucks, Pike Place all that. Crowded and crap. Criminal drug addicts (“homeless”) are all over downtown, and law is usually not enforced (not the cops fault), so just avoid it all.
If you want quiet and secluded hikes, drive into the Cascades, well away from the 90.
Enjoy your stay, anon.