Any good ones that you've encountered? Worth the time investment during a 2 day stop in Siena?
We spent one full day with Tours by Roberto. He took us (and 6 others people) on a combined history/archaeology/wine tour. He also does a full day in Chianti. Well worth the time!
I second Tours by Roberto. I used him for a tour on each of my past two trips to Italy and have another one booked with him this September.