The Museum of Flight was a pleasant surprise. It's a big, open building with huge, windowed galleries, located right by Boeing Field and close to SeaTac airport. Despite the name, the emphasis here is overwhelmingly on airplanes, with a few small displays on spacecraft, kites and gliders. They do a great job showing off their impressive collection of early and more modern aircraft, both civilian and military. The most impressive display is in the great gallery, with dozens of aircraft suspended from the ceiling and more on the ground. There's currently a display on hydroplanes there, as well.
There's enough here indoors and outside (you can board a Concorde and an Air Force One) for half a day's exploring, if you have any interest at all in aviation and aircraft. It's kid-friendly; probably more so for older children.
Admission is free one Thursday night a month; otherwise, if you're going with 4 or more people (or more than once), it pays to buy a membership.