One of my favorite stays in Europe was in Florence's Pensione Annalena in the Oltrarno section of the City (across the river from the center across Via Romana from the Pitti Palace and only a 10 minute walk to the Ponte Vecchio and 15 to the Duomo).
Very quiet 16th century palace-turned-convent-turned-girls' school-turned hotel, overlooks a lush garden. Our room had 14-foot ceilings and a huge bathroom by European standards, excellent service, and was very clean. Not luxury, but first class and relatively inexpensive:
http://www.hotelannalena.it/inglese.html
Have a great trip, and DO NOT miss the main cathedral in Siena if you have not seen it and the Piccolomini Library inside! Spectacular!