Seattle is my favourite MLB park that I have been to. Have been to Toronto, Chicago both, Seattle, LA, San Fran, Oakland, Anaheim.
The Boeing plant is probably pretty cool if you are into planes. It wasn't really my thing though. Apparently it is the biggest building in the world. Its definitely a day trip as its well outside of Seattle.
The one thing in Seattle I would absolutely recommend that you do is the Underground Tour. It is the best tour I have been on in any city and have done it three times. You learn a lot about the history of the city and its very entertaining.
The Experience Music Project and Sci Fi museum are near the space needle and the same ticket gets you into both. I definitely found that a worthwhile visit as well.