We just returned from a fabulous two week vacation in Italy. Part of the dream trip was to ride in Tuscany and stop at wineries along the way. At the suggestion of others on this forum, we decided to stay in Siena as our base for Siena, two days of bike riding, and a day trip to Florence.
Using Siena as a base for our bike ride was fantastic! After getting out of town (use a GPS for ease of not well marked streets), the ride was fantastic. The first day we did a 32 mile trek through wonderful rolling hills, stopping at several wineries including Felsina (a take or leave but a great little town) and San Felice (highly recommend this winery). Several other stops on the trek on small local wineries and olive oil locations (typically one in the same). On our second day, we did a shorter ride and went to San Giorgio a Lapi winery (and agritourismo). This is the winery tour you want to take. A small family owned winery where four generation lives and make the wine. Luca, one of the sons and the winemaker, gave us a personal tour, walked us through the vineyard and had us taste the grapes. He showed us the church on the property, built in the 1400s. One of the church frescos is the label on their Chianti Classico Reserva. A very nice wine which we ended up bringing back. On the way back from the church, Luca pulled some fruit off of their trees and had us taste the fruit - figs and loquats ! Delicious.
As for bicycles, we highly recommend DF Bike in Siena (http://www.dfbike.it/en/). Francesco delivered the bikes directly to our Siena B&B. The bikes were great (better than what I have at home). We rented the Focus Cayo road bikes. All carbon frame, Ultegra gearing, comfortable saddle. Based on the information we gave Francesco, they were already sized correctly for us. The rental came with helmets, repair kits and tools, and water bottles. Before our arrival, Francesco asked about the peddles we wanted. We didn't want to pack our bike shoes so we just asked for regular pedals and they worked fine.
As for the bike routes, we used google maps, did a search on wineries, and crafted our routes. We used the hiking option to get back roads that worked great. You can view the map and make sure the roads are paved. There are other options out there as well. If interested in the map of our first ride, let me know.