Add the dams along the Columbia River, and Dry Falls (although these might be too far out from Seattle and will be all-day expeditions out into the wilds of central Washington
)...
Deception Pass, at the north end of Whidbey Island. Great bridge views (from it, under it, along the arches, it's just a really cool place!).
Boeing Field in Seattle, and the aviation museum there.
Steve gave you a pretty darn complete listing of the top spots to hit, though.
Jim (born & raised in Seattle before Uncle Sam transplanted me)