San Gimignano used to be a pretty powerful town in Tuscany. It used to be a major cloth producer, which explains the many towers inside the city walls. Long time ago, cloth would be hung from these towers. Nowadays, only about thirteen towers remain. San Gimignano is a hilly town, lots of uphill and downhill cobblestone roads. But don't let this deter you. Part of the fun of exploring the town is getting yourself lost. Far from the main piazzas, you'll see the real San Gimignano - quiet, residential, and all so charmingly REAL.
During the day, the town is attacked by visiting daytrippers. They swarm around the Piazza della Cisterna (where the well is) and the Piazza della Duomo (where the church is). At dusk and in the evening, when they all leave, San Gimignano echoes with medieval magic. And early in the morning, its eerily deserted, save for the occassional street sweeper. Check out the city during these times - you won't regret it.
There are some spectacular views of the surrounding Tuscan hillside. Follow the map given by the tourist office. One of the more easy-to-find ones is just outside the main gate. Sit on the ledge with a bottle of Chianti and think again how very lucky you are to be here. I did that probably about a MILLION times on my trek across Europe. And a lot of those moments happened in Italy.
The other gate, Porto San Mateo, isn't as tourist-travelled. Just across the street from the gate is a sitting area which provides for a spectacular Tuscan sunset. I never realized that you can actually SEE the sun set - that you can actually see it going down - and it happens so fast. There are many benches there, as well as a payphone (in case you want to call a special someone back home and share the moment, or you may just want to call home and make everyone JEALOUS).