Maybe something off the well traveled road. Pisa?
Pisa ? less well travelled ? It's one of Italy's biggest tourist attractions.
Much depends on how long you've got and what transport options are available.
As francois recommends Lucca is a gem of an Italian renaissance walled town.
Florence ( more correctly Firenze ) is also well worth a visit.
If you have more time and your own transport, driving to Sienna via San Gimignano takes you through some beautiful Tuscan scenery.