San Gimignano and Volterra are the usual suspects concerning hilltop villages in the area. They aren't that high up though and I'm not sure it will be that much cooler and (especially the first) will be full of tourists.

Otherwise you could do as the Italians do and head towards the sea or you might as well go into the mountains north of Lucca (but that requires you have a car).

San Gimignano and Volterra are the usual suspects concerning hilltop villages in the area. They aren't that high up though and I'm not sure it will be that much cooler and (especially the first) will be full of tourists.

Otherwise you could do as the Italians do and head towards the sea. If you want to escape the heat in the altitude your best guess is the "Alpi Apuane" north of Lucca (but that is best done with a car).