If you go to cruisecritic boards and read the West Coast Departures board, there is a ton of great info.
You can take Amtrak train, Amtrak bus, rent a car or quickcoach shuttle. All options except rental car are a 4 hr trip. We prefer to rent a car but one-way rental from Budget was $300 from SEA for June 8, 2013 for 48 hrs with coupons. I priced an off-airport location for a little less. With quickcoach, you typically have to exit the bus at the border and go thru customs. The driver will handle your luggage but it has to come off the bus and then be put back on. They don't have their summer schedule yet but price is $55 one way and it makes several stops depending on your final destination.
We chose Amtrak train for $27/pp which has a dining car and no stops at the border. Plus we are looking at it as an "excursion". Sit on the left side if you are traveling north for best views.
Once you get to Vancouver on Amtrak, you will have to decide how to get to your final destination...cab, bus, etc. The station is very close to the Canada Place - 1.5 miles.