Cinque Terre is also really beautiful and interesting.  However, Florence was more memorable for my group.  We used Walks of Italy to book a guided walking tour for the Statue of David (and also for other parts of Italy).  The guides for the Statue of David, the Vatican, the Colosseum were all really amazing.  Here is a link.

The Tower of Pisa was only mildly interesting to us.  With the benefit of hindsight, we would have preferred to go straight to Florence and spend more time there.