I have acommodations near the Abbey S'Antimo on one night, plan to walk to Bagno Vignoni with acommodations on the third night, and don't know what to do for the second night. Any suggestions?
You could walk to Montalcino.
If I remember rightly, I think you're coming from Montalcino, aren't you, after Buonconvento? But several other ideas including taking the dirt track to Sant'Angelo in Colle, a very pretty little village on the edge of Maremma with some excellent options for lunch. Alternatively, going the other way you could walk down to Montepulciano Stazione and perhaps up the other side of the valley? Another lovely walk through vineyards and gorgeous scenery.