You should take Sound Transit's Central Link Light Rail from SeaTac to University Station, which will put you close to downtown and places like Pike Place Market as well as the city's excellent Seattle Art Museum. I also think Bill Spiedel's Underground Tour is quite fun, if a bit touristy. If you still have some time and the weather's, you might want to head out to the Space Needle, though I'd be inclined to skip that for just strolling and enjoying the downtown area.