Take a ferry over to Bainbridge Island and back. The ferry ride itself is fun. If you time it – you can walk from the Bainbridge ferry station into the “town” for lunch- there are some decent cafe’s there. (But it’s a small town. 🙂 )
Explore Pikes Place Market and Pioneer Square. I love both places. Watch them throw fish. Look for souvenirs at the little shops in the Market, rub the pig’s head. (Note – there are several levels of the market… tourists sometimes miss the lower levels.)
Check out the Seattle Art Museum. Laugh at or enjoy (depending on your perspective) the hammering man ouside the museum. It’s on 1st – halfway between Pikes Place and Pioneer Square. I love the architecture of this place.
Take the monorail from Westlake Center to the Space Needle. Go up in the Space Needle.
I love Seattle. I lived in Bellingham for 3 years and had friends in Seattle so I spent a lot of weekends there.
Skip the car – if you stay downtown you can walk to everything I suggested.