We just returned two weeks ago. Our situation was a bit different. We took 35 people from Church for Holy Week services. We were able to use the Metro to get around the city and see all the sites we were interested in. There is also a "tour" bus run by the city bus system. It has a guided commentary along the way. The benefit is that you can get off and on to actually visit the places you want. It's also airconditioned.
I echo the suggestion not to get a rental car. You can easily get to Florence and Pisa via train.