Siena is a city for exploring in foot. It's small – if a little confusing given that the streets tend to circle around the central piazza, and cars are not allowed in the historic center. Begin by orienting yourself in the heart of the city: the Piazza del Campo. Admire the uniformity of the palazzos ringing the square, the marble fountain Fonte Gaia, and the magnificent city hall, Palazzo Pubblico and its bell tower Torre del Mangia.
Siena has one of Italy's best art galleries: Pinacoteca Nazionale. It's in the 15th century Palazzo Buonsignori and has some of the best Sienese art of the 14th and 15th centuries, as well as a vast collection of paintings from northern Europe, including Durer. There is also an extensive sculpture collection.
Tuscany is not huge but it is rich in history and beauty. If you've ever had enough of what Siena itself has to offer, think about a day trip to Florence to see the differences between these arch rivals in art.