It doesn't look far on a map but it's a good 90 mins / 2 hr drive as the roads are very slow and windy. You must visit Cortona (and that's not far from where you'll be) but don't include it on the day on which you visit Pienza for it deserves a full half day at least and you won't have time. With enough time, you could make a good circular route by coming over to the Val d'Orcia via the Perugia / Bettolle road exiting at the Val di Chiana exit. From here, Pienza is signposted and the road will take you through beautiful countryside bringing you out eventually just west of Montepulciano on the main Pienza road, which is 5 mins down the road to the right. Visit Pienza and if the weather is kind to you, lunch outside with a view would be great. Il Chiostro now has competition for a terrace restaurant with a view, La Terrazza, and it's far cheaper too. (It's signposted from the public gardens.) Once you've visited Pienza, head off in the direction of San Quirico to catch some of the iconic Tuscan postcard scenes. The famous farmhouse with the cypress is very shortly on the right just as you leave the village, 5 mins later way over to your left on a hill you'll see la Vitaleta that Lynn refers to. Stop at San Quirico, the official heart of the Val d'Orcia and a much overlooked place. If time permits, head on to Montalcino (it's about 15 mins), otherwise, retrace your steps back towards Montepulciano perhaps stopping for wine tasting along the way (there are numerous places right on the road). To get home either go on to Chiusi and round Lago di Trasimeno or head through the Montepulciano vineyards and back via the Val di Chiana villages of Acquaviva, etc. I should stress that this is merely the tip of the iceberg and visitors could quite happily spend an entire two weeks or more touring this area and still not see everything.