My favorite place for plane watching is at the King County Airport, most often known as Boeing field. This is the Airport the Boeing uses for testing planes as well as painting many of them.
It's always entertaining to drive along the field and try to guess what all of the tail paint jobs represent. It was also entertaining when there were only five 777s to see the entire production run sitting side by side.
This field is also where the old warbirds and newer military planes use when they come into town. It's pretty cool to see a B-52 and a Stealth fly in on the same day.