The 13th century Palazzo Publico, otherwise known as Town Hall, towers over the Piazza del Campo. Still home to Siena's town officials, it's a pretty impressive building that also houses an interesting art museum, Museo Civico, in case you haven't seen enough frescoes yet. I remember some of the allegorical artwork in that museum to be particularly creepy, but it was fun wandering around this huge castle.
Though I can't remember the name of it, there is a nice little restaurant behind the Palazzo Publico that is worth checking out. Both times I ate there the food was great and the service was quick! Amazing!!