Depends on what you're into. Weather is kind of turning to crap, but driving up one of the Passes and dicking around in the mountain could be fun the "Cascade loop" has some wonderful scenery. Going out to Alki as mentioned in one of the other posts linked to above is nice as well. Pikes place market downtown is a standard thing to do, and lunch at Kells (big Irish bar up the street in the alley) is always pleasant. Ballard is a hopping little town, and normally has places playing music, and great breakfast and lunch places (spendy, but SUPER good). If you like BBQ pork sammiches, go to Paseo's. Literally world famous, people actually fly to Seattle to eat there (BBQ cuban sammis) although I sort of prefer Royal Grinder in freemont (down the street from Ballard).
Boeing Museum of flight has a LOT to look at, and I don't know the status, but the new flight museum at Pain Field is supposed to have a lot of Paul Allens stuff (First time I saw a F-5F Tigercat, PBY AND A-1 Skyraider in real life AND got to watch them fly!!! aeronautical boner material!).
Lots of other random Museums around also, I guess google and see if there's anything that strike your fancy.
Redmond has some cool little attractions. The Three Lions pub is my favorite little place, English bar. Half-price books if you want to pick up cheap reading material, Nike park, although you can't tell anymore, used to be a Nike missle site (S-13 and S-14, double site of Nike-Ajax missiles).
Could always head north if you have the time and check out places like Camp Casey, the coastal defence forts, pretty damn cool and this time of year in the grey drizzle, has a lot of atmosphere.