Coming from SeaTac you can follow I-5 to Blaine and cross on WA-543 - BC-15 to the Trans-Canada and connect to the Whistler highway, avoiding all of downtown Vancouver, which you would not do from YVR. The downtown and Lion's Gate Bridge are the main traps. The suggested route uses the new (2012) Port Mann toll bridge (https://www.treo.ca/the-bridge/) and you should find out how to register to pay in advance - otherwise the plates will be photographed and a bill sent to the rental car company with extra fees.