The city of Siena is famous for its main church The Duomo, for the piazza where every summer they hold the horse race and for the surrounding hills where the best chianti is made. It is a rather small and walkable city.
Great to stroll around, grab a gelato and enjoy the medieval street. It can be quite busy in the middle of the day I suggest to get there early in the morning or later in the evening. A visit inside the duomo is a must, the mosaic floor are incredible.