Using YVR's handy connections tool, you can put in that you're arriving internationally, changing to fly to the US, and both flights are on Air Canada.
Arrive at YVR
Most passengers will have their luggage checked-through to their final
destination, so you will not claim your checked-bags at YVR. Please
ensure you have a boarding pass and baggage claim tags for your
connecting flight. if you do not, contact your airline for
instructions, you may have to check-in again.
Follow signs to USA Connections.
More details about US customs and the like exist on the page for you to peruse.
Note however, that if your flight arrives before 7.30am, or after 6.30pm, there is a different procedure, also listed on that page.
As for departing the airport, yes, this is possible - a friend did this on his connecting Air Canada flight - came out to meet me for a few hours. The downtown core is about 35 min away by Skytrain. I recommend this other question - How to visit Capilano Suspension Bridge Park and Stanley Park from Vancouver Airport? which has details on getting in from the airport to see a few of the sights, and this question shows the various costs of transport, if you want to consider something other than the very convenient, clean and safe Skytrain.