What are the flight times,
@Happy Dude? If I look at UA economy award space on Easter Sunday (for example), the MEL-SFO flight arrives 07:05 and the SFO-CDG flight departs 14:40.
Assuming a couple of hours to clear immigration/customs then redrop your bags, and wanting to be back at the airport by probably 13:00 at the latest, you could still take BART or an Uber into town and see a couple of things. It wouldn't be the full San Francisco Experience but you could walk along the Embarcadero and Fisherman's Wharf, maybe ride a cable car.