My wife and I are on the same cruise. We plan to do Rome and Nice/Monaco on our own. We also have a rental car reserved in Livorno and plan to take in the Tuscany countryside and villages rather than going to Florence. What kind of interests did you have ?
We do want to see Florence. I was in Italy for a month in my 20's and have already seen much of Tuscany. Particulary liked San Gigminano and Voltera.
In Rome we planned to do the Rome on your Own excursion. Take the ship bus in and try and see St. Peters, but not the museum because of time, the Colleseum, Old Rome the Forum, Pantheon, Spanish Steps and Trevi Fountain and Piazza Navona.
If your interested in something similar maybe we can share taxis around the city as we think that will be the fastest way.