The Everett Boeing tour is a lot of fun. It may be too much of a trip for a layover, though. The other problem is that you can only get tickets on the same day, so you may arrive in Everett at 10 am and not get to go on the tour until 12 or 1 pm. More info is at http://www.boeing.com/companyoffices/aboutus/tours/

Also, you may want to take the (shorter) trip to the Museum of Flight. The website http://www.museumofflight.org/ indicates that you can get there by taking bus 174 from SeaTac. It's a worthwhile museum if you like planes and aviation history (especially Boeing history). It's located on Boeing field, and you may see an interesting flight (I happened to see an early 737-700 test flight, and the delivery flight of a Qantas 747 on two different trips there).