Let me just make a minor adjustment to Priscilla's excellent post. With a 24-hour pass, you do not need to do it all in one day. You can spread it over two days if you plan it right.
We activated our pass at 1:00 pmnand went over to San Giorgio. Admired the church and the views, then rode the vaporetto to Giudecca and walked around a bit, then found a waterside place for lunch. We picked up a vaporetto going clockwise to Piazalle Roma and down the Grand Canal. We got off at Rialto to go home for a bit to freshen up. Then we walked to Fondamenta Nuova and caught a boat for Murano. We walked around the island, stopping in at a couple of famous glass places with no obligation because we were on our own. We timed the ride back to coincide with sunset which made for a spectacular ride.
The next day my husband went for a swim in the public pool in Cannareggio, then we caught the vaporetto at Sant'Alvise to go to Sant' Erasmo, the "garden island.". This is an island full of farms, and not much else. It makes for a nice getaway from the city, but not if your time is limited. We walked for hours, enjoying the sunshine and views, and then caught the vaporetto back to Venice. All within the 24-hour pass.