Buongiorno,
If you are going to take a couple of days to do this trip then I would train it to Orvieto, stay overnight in this great little town, and then rent a car.
I suggest that you also look at an itinerary that takes you on the back roads up the road called via Cassia. It is very panoramic and will take you by Lake Bolsena, Acquapendente, Radicofani,
Bagno San Filippo,
Montepucliano (including some of the great smaller towns like
Sarteano,
Cetona and
Chiusi). Then head over towards
Pienza, Bagno Vignoni and
San Quirico before ending up in Siena.
day 1 Oriveto - one day sleep there pick up car the next day
Day 2 Via Cassia - back roads maybe a stop at the thermal waters of Bagno San Filippo end in Montepulciano
Day 3 Montepulciano - Pienza (for views and shopping) Bagno Vignoni for lunch - San Quirico and end in Siena
Day 4 Siena
If you have just one day, I think you will be pushing it to include everything that you want to see. However, I would still recommend a car in Orvieto and a straight drive to Siena with a stop at Bagno Vignoni, Montalcino and then Siena.
Buon Viaggio
Donna Denise