If you like planes, there are two must-sees up here:
Boeing Everett Plant Tour, and
The Museum of Flight at Boeing Field, where you can not only check out an older Air Force One but also a Concorde. There's a lot to do there.
A note: if you are planning on spending a couple days in Seattle itself, you might decide to get your rental car only after branching out to other areas. The downtown hotels charge a steep parking fee, and the public transit is pretty good up here. Link light rail will be running into the city from the airport by that time, so instead of a day's car rental and parking fees you'll just be out $2.50 each for the rail if you plan to just check in and see the local sights your first day.
Have fun!