A few things:
If you want to take a ferry boat trip, don't pay for a touristy boat. Take the Washington State (public transpo) Ferry over to Bainbridge Island. Ferries leave every 1/2 hour - 45 minutes and the boat ride is about 1/2 hour as I recall. I'm thinking it cost seven or eight dollars total.....you only pay for the outbound trip, and get on the return boat whenever you like. Bainbridge doesn't have a lot to do but the boat ride is pretty, and a steal for eight bucks!
Pike Place Market -- just wander around and look at stuff. A HUGE farmer's market.
There is a Museum of Scandinavian History in one of the Seattle suburbs that I liked.....but probably wouldn't mean much to a non-Scandinavian
Seattle Art Museum is wonderful if you're into art.
There is a museum by the Space Needle that's a combination Science Fiction and Grunge Music. Overpriced for what you get, IMHO, but something to do if that's an interest.