If you decide on Amtrak, the tram will take you directly from Seatac to the King Street Station in a half hour, It's pretty much the only easy public transit trip in Seattle (which works well for me, I work for a towing company here ? )
As for getting around Vancouver, Car2go car rental is also an option. There app shows you the nearest car and unlocks the door for you, and you can just leave it in any legal public parking spot inside their coverage area, charges by the minute you are actually driving the car (Mercedes, since that's who owns the company)(I know a lot of their employees by name, a lot of part time drivers seem to have a problem with the term "legal public parking spot"), there's a couple of other companies in Seattle, Reach Now(BMW) and Lime, don't know if they operate in Vancouver also.