There is baggage storage available in the international and domestic terminals:

http://www.yvr.ca/en/passengers/shop...d-services/276

You can take the Canada Line rapid transit from the terminal into the city and back to the airport, ~30 minutes each way. The entrance to the station is on the departures level between the domestic and international terminals. I suggest you go to the end of the line, Waterfront Station, where it is an easy walk to Canada Place and the shoreline walkway, great views of the harbour and mountains.

Plan on arriving back at YVR at least two hours before your flight to check bags, clear security and US customs and immigration.