My dad and I are interested in visiting Siena on our upcoming 12-day trip to Italy (12/11-12/24). Should we stop there on the way from Rome to Florence, spend the night, and head to Florence the next evening, or spend an extra night in Florence and do Siena as a day trip?

From the standpoint of minimizing travel time, it seems more sensible to take the ES train from Rome to Florence and do Siena as a daytrip. I guess the question is whether we'll have enough time to see everything we ought to see inside the walls if we don't make it to Siena until, say 10 or 10:30am (vs. an 8:30 or 9am start if we spent the night).

ETA: Finally, is it possible to to Pisa and Siena in a day without getting too exhausted?