Ciao wpy,
You can do the ring road around Rome and head up on the A1.
I recommend a stop in Orvieto to visit and for an early lunch.
Then at Chiusi Scalo, leave the A1 and head toward Montepulciano.
I'd recommend a stop in Pienza but most of all, enjoy the scenic drive through the Val d'Orcia.
If you have a chance to come back for a day trip in the area once you're in Siena, I'd recommend visiting Montalcino and Montepulciano, too. The whole area is beautiful and just pleasure to spend time in and enjoy - stupendous views and food and wine!