Oke, first is Paine Field.
Two years ago I did a guided tour at the Boeing factory here. Very interesting, but cameras and binoculars are not allowed.
The parking lot of the Future Of Flight centre is an excellent spot for arriving and departing aircraft if runway 16 is in use. Light is good from around 15.00h till sunset. No stairs needed
. A 70-200/70-300 lense is ideal here, for the airliners. When I was there, on a Friday afternoon, there was not that much traffic.
Boeing Field was certainly a lot busier than Paine Field. I was there on a Saturday all day long and the amount of bizjets was huge. Didn't see much Boeing traffic flying that Saturday, but to my surprise I catched some military traffic on a late Saturday afternoon (Vance Texans and Columbus Jayhawks).
I stood all day at the parking lot of the Museum of Flight. Unfortunately the light is only good after 15.00h. 300mm is the way to go here and you need stairs for the 2-2,5 meter high fence. If you don't have stairs (I didn't have them neither), you can walk back a long way in order to shoot over the fence, but this is not ideal. For more spotting places at BFI I recommend this spotting guide:
http://www.plane-spotter.com/Airports/U ... de_BFI.pdf
Hope this helps. Have a great trip!