Work are sending me to a conference in Portland, Oregon next week, and I thought I'd make the most of the free flights to spend some time in Seattle too. I have four nights in each, with time for sightseeing in Seattle during the day. Anybody got any tips for things to do/see?

I'm perfectly aware this is a not-very-thinly-veiled "I'm going somewhere pretty good for free" thread, but I'm comfortable with that. I'm incredibly excited.