Though it is not a big city, Siena really deserves at least one full day and its central location in Tuscany makes Siena to be one of the overnight stops if you plan a tour in the region.
I definitely recommend you to go straight into the heart of the city, to the beautiful Piazza del Campo, one of the landmarks of the city. Truly unique with its shell shape, for its bricks pavement and for all of the buildings facing on it.
Among the buildings overlooking Piazza del Campo, I recommend you the Palazzo Comunale, in brick and marble, and the Civic Museum with several masterpieces of Sienese artists, including the beautiful Majesty by Simone Martini and the cycle of frescoes called “Good and Bad Government” by Ambrogio Lorenzetti.
The Torre del Mangia, with its height of 87 meters, offers one of the most beautiful views of the city.
The Cathedral in Siena is one of the most outstanding examples of Romanesque-Gothic siena duomochurch in Italy and fascinates even from the outside. Inside works signed by Donatello, Nicola Pisano, Michelangelo and Pinturicchio.
The interior, like the exterior, is decorated with white and dark green and the pavement consists of 56 wonderful engraved squares in marble.
Save some time to just stroll through the streets of the city, browse the shops, wander through the colors and emblems of the of the city.
If you still have time my advice is to have a tour of the surrounding area to Siena e Montalcino, to the Chianti and to San Gimignano.