Here's my opinion:
Qwest Field and Safeco Field - who cares, they're football/baseball stadiums.
Pike's Place - a bunch of crappy shops with some people throwing overpriced fish.
Museum of Flight - huge rip off. Not even in the same category as other museums that are free (like the USAF museum in Dayton). I'm an aerospace engineer and pretty passionate about airplanes, and the Museum of Flight sucks (and the old Air Force One they have is not the one Kennedy's body was on).
Green Lake - it's just a small, overcrowded lake.
Space needle - actually not too bad. It'll give you a nice view of the area, but in the winter it'll be overcast and you probably won't see much.
I've never been to the EMP, but I do want to check out the sci-fi museum.
Library - I thought it was interesting, but would only take a couple hours of your time at most.
The best part of Seattle is what's around it - the Cascades mountains, Olympic mountains, Olympic coast, and Puget Sound. Unfortunately winter is a really bad time to see the mountains (unless you want to get killed in an avalanche). You can still look around the Sound. A trip to the coast still might work if you have time (I believe it's the least developed coast in the continental US, but I could be wrong). A drive through Whidbey island would be interesting.
I would just drive around all the different parts of the city and its suburbs. It might also be fun to hop on a ferry to somewhere.
Just my opinion though