I stayed at this b&b in Siena a few years ago and really liked it-

http://www.residenzadepoca.it/en/offerte.php

My room was very nice (large too), the hosts were great, and the location cannot be more central.

It is possible to take the train to Chiusi (with beautiful views out the window) and transfer to a bus from Chiusi to Montepulciano. The bus coordinates departures with the train arrival. It is about 45 minutes on the bus. When returning, take the bus all the way to Siena and you will travel through beautiful countryside. Double check with the Montepulciano bus ticket person on the departure time for the Siena bus. Sometimes the times posted are not accurate.