If not San Gimignano than ...

Volterra

Volterra is not so well known as San Gimignano. But definitely it is worth a visit.

It is equally interesting but less crowded. The town is located on the top of hill with a wonderful view over central part of Tuscany.  It is a little bit out of the main tourist trail. The town is known for its alabaster and salt, and its history goes back to the Etruscan. You can find the remains of Etruscan epoch such as fortified walls, Porta dell 'Arco, aqueduct and several burial places.  Expositions of the artefacts from that period are held in the Palazzo Tagani and in the Museum Guarnacci.

In recent years, the town became known as a place of the saga about vampires "New Moon" by Stephanie Meyer.  It is the second part of the saga "Twilight". It is said that the town is experiencing an invasion of tourists in love with the characters.  But we did not meet too many of them in the middle of the holiday season.  Maybe they preferred touring the town after dark.
You will find more information at the dedicated website about Volterra.