With half a day, you have enough time to stroll through the historic city center, climb Torre Grossa or Torre Salvucci Maggiore for the view, visit the Duomo di San Gimignano, do a little shopping, and have lunch or dinner. With more time, venture away from the main streets to explore the side streets, visit Parco della Rocca, go wine tasting, visit San Gimignano 1300, and try a second restaurant in town.