I'm taking a little trip, and don't know if there's any fun Class III to be had in or around Seattle. Is it worth figuring out how to get a boat out there for a few days? I'm flying, so it's a tough question!
AquaSports in Redmond will rent you a kayak for the day, that'd be easier and probably cheaper than flying yours out.
Not much running right now -- the Skykomish is at a good level and a fun III-IV run, the Sauk (III) and the Powerhouse section of the Snoqualmie (II+) are also possibilities. Or head to the ocean for some kayak surfing.