I don't know if you will have a car, but the Museum of Glass in Tacoma is really, really cool. I think it's maybe a 45 minute drive or so from downtown Seattle. And, even though I'm not much of an airplane fanatic, I really like the Museum of Flight, which is near the airport. It's really well done and HUGE. You could do both in one day, as they're both right of I-5, so you don't have to go out of your way.