I wouldn't attempt the trip from port to Florence on my own: the traffic is terrible, and if you pay for the ship bus, you'll be sure of getting back to the ship on time. It took 45 minutes to get through the rush hour traffic out of Florence.
St. Mark's Square was unbelievably crowded, so we headed off down alleys and side-canals which get less touristy the farther you go. We had a pizza lunch that cost about 60 DOLLARS (with a beer each), and decided against a gelato shop that offered scoops starting at 11 euros. The prices are crazy, but that's just part of the experience.
We walked across the Grand Canal to the Peggy Guggenheim Museum, but saw only the very pleasing shop, not the museum, due to crowds.