The Museum of Flight is just junk. SEriously.
You can take a walk around a 1960's-era Airforce One, which is even filled with the obligatory cheesy-manequins-positioned-at-the-desk-to-look-like-they're-working things.
There's alot of airplanes hanging from the roof, and you can see alot of mockups of different things, but it's really really a bad museum. Even my husband who is enthralled by anything dealing with airplanes was begging me to leave.
Wanna see planes for cheap? Hang out on E. Marginal and watch the planes going to and fro Boeing field. You'll save the cash AND see more interesting things than you could have in the museum.