Just came back from a brief holiday in Italy. I've spent a couple of days in Tuscany (as last year - when I went in Siena) and this time I visited San Gimignano and Firenze. I never been to the former, but I went once in Firenze when I was in the high school, and never back since. I have to say that going back armed with a reflex camera was totally another story and I enjoyed taking some nice pics of the two towns.
San Gimignano in particular was quite charming: a small medieval / pre-renaissance with protruding towers all around the place. You can clearly spot it from the highway when you are still miles away. The town is quite small and intimate, you can enjoy some fine wine in a very calm atmosphere.
There isn't much I can say about Firenze ... it's already famous and well known by everyone! Just one advice: do absolutely go and visit Piazzale Michelangelo, the best view of the city is waiting you there. Or you can also climb the Cathedral's dome, if you don't mind over 460 steps!