For transportation from SeaTac to the Cruise Ship terminal try the van service Shuttle Express. You can contact them via their Web site. I think they are much less expensive than a cab for an individual. For 2 or more people a cab may be a better deal. The true cheap way to get downtown is either Metro Transit or Sound Transit.
The Edgewater hotel is right by the terminal, so they would certainly be able to take you there.
The Seattle Marriott Waterfront should be across the street. I should have looked when I was down there yesterday, but I was too busy looking at the NCL Sun. Note that there are several Marriotts around Seattle.
There are plenty of things to do in Seattle. You can go to Pike Place market to shop and look around. Westlake Center and the whole shopping district is a short cab or bus ride away. The Pacific Science center has science type exhibits, and an Imax theater. There is another Imax theater somewhere on the waterfront too. You could ride the waterfront trolley to Pioneer Square. Near Pioneer Square is the football and baseball stadiums. There are numerous art galleries, and other old town type shops.