Sienna to Firenze (Florance) by SITA bus. Although the highways are not that great quality-wise, the trip is rather cheap (€7,50 one way). It is actually the preferable choice of transport if you want to go to Siena; taking the train requires a change on a local bus at the station and getting the right line is somewhat difficult. The SITA (and other highway buses) all arrive and depart near the actual town center and are therefore much more convenient.
Halfway through the weather turned grim and provided with a downpour and some hail: thans to the upper deck, fron row seat the view of such weather is quite amazing.