Rentals in Sienna
Personally have not rented there before though.
I rent in Florence
Here
Some pointers:
Decide if you want a scooter or a motorcycle.
A 450CC or better motorcycle works just fine on the Main Highways if you get lost and need to use one to get back. The little scooters are restricted to the back roads. You'll want at least a 100-150CC scooter. Italians on other Scooters pass on both sides. So check that right hand mirror before you do anything. If coming to a blind bend in the road, and you hear two short Horn signals, slow to a stop. A bus or a truck is about to come through and will take the entire road. Pull over a lot and let the locals pass you. Outside of major cities, such as Florence, the roads are very peaceful and the traffic is light.
Road signs, particularly arrows pointing to a destination at a fork in the road can be entirely misleading. So its good to have an idea of where you are at all times in relation to the back ground terrain and maintaining a sense of North/South/East/West. Never hurts to ask a local if you are unsure which road to take.
You'll need full eye protection, not just sunglasses due to the volcanic dust and pollen.
A suggested route. You can do the entire route virtually by looking at google sat. photos.
Head a little South of Sienna then take E 78 East. Turns into 326 till you get to Sinalunga. Now head to Piano. Piano to Gracciano. Gracciano to Montepulciano. Montepulciano to Pienza. Pienza to Montalcino. Montalcino to Buonconvento. Buonconvento to Asciano. Then take road 438 back to the E 78 and Sienna.