Just found Rick Steves Florence and Tuscany guide book. You can find extensive advice in his books but I would advise seeing Accademia(David), the Duomo, and the Ponte Vecchio. These are absolute musts. Then if you have time either the Bargello Museum (Best Statues) or the Uffizi Gallery (Finest Paintings). Make sure you stop for gelato often and maybe a good lunch. When going to Accademia or Uffizi make sure you reserve tickets in advance as they are very crowded and I am sure you do not want to waste your precious time waiting in line for an hour. Make sure to call 055-294-883 a few days in advance or pay www.weekendafirenze.it
to do it for you. Have a great time.