I agree 100% with BearX220's suggestions.
My wife is from Seattle so when we get back there the kids love the Ballard Locks, Museum of Flight, Woodland Park Zoo, and lunch at Spud's on Alki beach (West Seattle). I'd also suggest a day trip to Mt. Rainier south of Tacoma, the Tacoma Narrows bridge, and a return ride on one of the Ferry's is pretty cool to them. Might seem strange, but they like just walking around the water front on Elliot Bay.