One day is not much time. I have been on 3 previous cruises out of Vancouver and going again July 28th. I would spend time on in Harbor area and Gastown. Take the ferry across the Harbor. Very cheap and a small shopping center on the other side. Eat dinner near Robson. Check out the underground shopping mall in downtown.
I would take the Trolley around town and get off at the stops, see what you want to see, and then hop back on another trolley. That way you will see a brief tour of the downtown area including Stanley Park, Gastown, Chinatown, Granville Island, etc. Another choice is to take the Ferry across the bay and go to Capilano Suspension Bridge and Grouse Mountain.
We are a family of four, so we spent a lovely day at Stanley Park (its huge) and the aquarium there.
Another great day would be to go to Sewell's marina and go on a fishing charter, or take the sea safari.
Van Couver is a beautiful city. Oh, we also loved Grouse Mountain. Sorry, one day is not enough! You'll have to choose!