Siena is a great day trip from Florence and still has many of the towers intact that the Medicis had removed from Florence, so it gives you more of the flavour of what Florence looked like in its prime.

But the real place to check out when you’re in the neighbourhood of Florence is Cinque Terre. Oh my God. Stunning and unique. If there is ANY way for you to go, GO.

http://en.wikipedia.org/wiki/Cinque_Terre

P.S. I think Pisa is totally forgettable. Go for Cinque Terre and Siena first.