Neil, my better half goes back and forth to Seattle a fair bit. Whilst her priorities were not motor racing, she's visited the Seattle Museum of Flight
http://www.museumofflight.org/ which was, apparently, well worth a visit.
Other Seattle area places to visit include the Boeing factory at Everett
http://www.boeing.com/companyoffices/aboutus/tours/and the Hydroplane and Raceboat Museum
http://www.thunderboats.org/ - Lake Washington was used from the 30's to the 70's for hydroplane races (multi engined monsters such as Gar Wood's Miss America boats and Slo-Mo-Shun)