Not sure if Seattle offers it, but in some airports there is a luggage check area where you can drop your luggage for a fee. As queenofcruizin states, there are some hotels that will offer "day" rates. Or, if you're going to rent a car, you can throw the luggage in the trunk. In your case, more like a van
I'd really think with the popularity of cruises out of Seattle, their airport would offer some type of luggage service.