hi
especially SeaTac, King Country and Renton are very easy to combine on a day.
We spend several days in Seattle and did several hit/run visits at Renton and King Country. All 3 airports are less than 30 minutes apart from each other. You have to be lucky to see anything fly at the Boeing fields, but a lot can be seen/photographed from the outside.
(we were lucky though with a 787, 747-8F, and Wedgetail doing testflights
)
Paine Field is a bit further away, but definately worth a visit also for the factory tour. Paine Field was packed with 747-8's and 787's, and lots of decent photo opportunities on those as they are all over the place on taxiways and new build hardstands.
rgds,Koen