It is called the Manhattan of the Middle Ages because of its towers, but it so much more. It's one of the best preserved medieval towns. Little and charming, especially at sunset:)

A lovely route to explore: Volterra, San Gimignano, Siena. Can fit into a day trip but you may have to rush them. Rent a car for this - it's the best way to go about it. There are lovely little villages all around the area and you may feel like stopping more than a bus or train would allow you to. Plus: the scenic route is by far in a car.

As far as public transport goes: to San Gimignano you can get from Florence only by bus (SITA bus), there is no train. The ticket can be bought at the SITA station near the SMN train station and it covers the entire distance even though you will need to change teh bus at Poggibonsi (so no second ticket needed).