Chiusi to Siena

For our first Italian trip, in September, we will be staying near Chiusi (renting a car at fco for a week), then turning in the car and ending with three days in Rome. From talking with friends and reading comments on this website, we are thinking of spending most of the week driving and walking around the Tuscan hill towns, and, rather than go to Florence for a day as we had orginally planned, going to Siena instead. We picture getting there early enough for the kind of breakfast experience Rick Steves has in the guidebook - (from a balcony overlooking Il Campo) and staying through late afternoon. Since we have a car, is driving the best way to go to and from Siena, or should we consider maybe just driving to Chiusi and taking a bus? That is if that is even a possibility? I've asked a few questions already in planning the trip and gotten wonderfully helpful responses, so hope you will weigh in on this one. Mainly wondering if it will be a hassle to find a penalty-free parking space and then deal with getting into town, and/or if a bus would get us pretty much where we want to be. (I assume a train would be too expensive multiplied times 4 - we will be two couples travelling together) I have had no luck finding information on bus schedules. Thanks -