I just got back from there and had a lifetime-memorable trip. Brought road shoes and pedals, but ended up renting alloy hybrids from an ancient shop in Siena. Was glad we did, because the granny ring and wider tires were well suited to the dirt road 20% grades we explored. It was a bit of a drag on the good pavement sections, but I wasn't racing.

Tips: You don't always need to ride back to the place you started from. The regional trains all accept bikes. Personal fares are very cheap, and the bike supplement costs €3.50 for a 24hr period. You just need to figure out schedules and locations.
I'd seriously recommend a triple. Especially if you want to ride up, for example, Montepulciano's main street at a non-lethal pace. me: