I most always drive, but did take Quick Shuttle once, no complaints. One way rentals are available, National, Avis, Hertz, Budget, Alamo. I have rented from them all except Hertz- going with the cheapest at the time.
You want DOWNTOWN Vancouver and not the airport. Easy to walk and sometimes is much faster than waiting for shuttles. There are some offices at Canada Place as well.
But allow 4 hours for the drive and if you are flying out of Seattle that day, don't plan a flight before 3pm if you haven't been there before. It is slow stop and go city driving out of Vancouver- have a good map and Seatac is 12 miles south of Seattle, can be VERY congested traffic. Gas is right outside the airport.