We rented a car, but the rental car was proving tricky... if we returned the rental car to the airport the day of the cruise, we'd then need to find shuttle or taxi service from the airport to pier 91....
I am just re-reading this thread now that I have done a lot more research about the cruise, and cruising in general, and want to read it again with fresh eyes. So sorry if I pop up and chatter a bunch all of a sudden while I re-process all that everyone has said here.
FOR PEOPLE RENTING A CAR IN SEATTLE--ADVICE FROM A LIFETIME SEATTLITE:
I would personally recommend renting a car downtown, rather than at the airport. The airport is 30+ minutes away from downtown (depending on traffic), and it isn't super practical to be downtown at your hotel, then drive back south to the airport to drop off your car, then take a shuttle back to the ship--embarkation day being the one day you are probably on the tightest schedule, with packing, checking out, wrangling bags (then returning a car, re-wrangling bags, taking a shuttle, re-wrangling bags at the terminal, etc). I would recommend you take a shuttle (your hotel may offer a free one) or the light rail or a taxi into downtown from the airport, then get your car rental downtown. That way your return is downtown as well. (That is, if you need a rental car at all**! See below)
Enterprise rentals will pick you up and drop you off within a reasonable radius, so you could return your car downtown and then they can drive you to the pier. I am sure other rental companies have their own shuttles and versions of this service they can offer.
**Ultimately, you may not NEED a car. Seattle has SO MUCH to offer in a small area--the city really isn't that large, but it is packed with awesomeness. Hubby and I recently did a "staycation-esque" trip where we spent a long weekend in a hotel downtown (this is about 10 minutes from our home). We parked our car in the hotel garage for the entire time and went to a show, took a food tour at the market (Pike Place Market really is awesome, even to us locals), went out to dinner at some restaurants we had always wanted to try, went shopping at boutiques, went to the Seattle Center (we adore the Science Center--a must see if you're geeks like us
. We spent an afternoon on the waterfront, where you could do more shopping, go to the aquarium, take a harbor tour (if you desire more boat time in your life), take the Duck tour, go on the Underground Seattle tour (another one I always highly recommend), go to the art museum, go to the EMP/Science Fiction Museum, ride the monorail, hit the arcade at Gameworks...the list goes on.
And ALL these things are in walking distance or a cheap and easy bus trip away from downtown hotels. A car really is a needless expense unless you have plans further away.
Anyway, wanted to throw that out there. I am going to drop this info in the official Seattle Port thread as well, as I think it might be of interest to folks coming into town.
And if you need any further advice or ideas from a local, I am....wait for it...ALL EARS!