Yes you can easily do these in one day - they are all close together. I would plan the trip around a lunch out in either Montalcino (David has a good recommendation here in the Restaurant List) or in Pienza. I would exchange Montepulciano for a vist to Monte Oliveto.
Monte Oliveto is in a lovely location and the Sodoma frescoes in the building attached to the church are incredible. There are about 16-20 panels on the 4 sides of a room which is really like a four sided hallway with an enclosed atrium in the middle.
Montepulciano is a nice town, but so are Pienza and Montalcino and they are closer together and closer to the main road down from Siena. Save Montepulciano for your next trip.
But working out the timing is tricky. Here is what I have come up with.
Sant'Antimo chanting: 12:45pm, 2:45pm, 7pm. If you aim for the 12:45 chanting, you can go just before lunch and won't have to rush lunch to get to the 2:45pm chanting. If you leave it until the 7pm chanting, it will be getting dark and you will want to be heading home.
Monte Oliveto hours: 9 - 12pm, 3:15pm - 6pm (no sleeveless tops, no shorts) I would see Monte Oliveto after lunch because it opens at 3:15 and the towns are closed down during this time - so spend the time at Monte Oliveto. With parking and walking to the abbey and visiting the gift shop run by the abbey, figure about 2 hours.
I have the schedules on the web site.
http://www.slowtrav.com/italy/todo/tuscany.htm#sant_antimo Also, if you are going to do wine tasting in Montalcino, it will probably be best to go there in the late afternoon won't it? I can't imagine wine tasting in the morning.
Proposed Day Trip ItineraryLeave Colle Lungo at a leisurly 9am (unless you are an early morning person, then you will have to redo this schedule). Drive straight to Pienza. Leave yourself about an hour to get there.
So, at 10pm you are in Pienza. Spend about an hour wandering around, visit the church, have coffee in the main piazza, get your International Herald Tribune at the book store on the piazza just outside the town walls - near the parking.
Maybe take a short drive from Pienza to Santa Anna in Campregna (I am not sure of the spelling, but you go from Pienza west back to the highway and you will see the turn off on your right - this is close to the house we are staying in just before you will be here). This is where they filmed The English Patient and it has a room of very understated Sodoma frescoes. There are light switches that are hard to find, but hunt around for them. It was deserted when we were there.
Or instead head to a farm where you can buy fresh pecorino. But you can also buy good things in Pienza. Some of the shops in Pienza and Montepulciano sell Pecorino that is not made locally, but in Sardinia. Have a look at my food shops list for some place we have gone. The Carla Capalbo book has good listings.
http://www.slowtrav.com/italy/food/shops_list.htm If you are an early morning person, you could start earlier and visit Montepulciano, then Pienza and skip Santa Anna.
Then drive over to Sant'Antimo and catch the 12:45 monks chanting. Allow about 30 minutes for the drive from Pienza. Make a lunch reservation in the area for 1:30. Sant'Antimo is only 10 minutes or so from Montalcino. Restaurants usually keep seating people until 2pm. Lunch in either Pienza or Montalcino or some other town close by would be good.
After lunch drive up to Monte Oliveto. If frescoes are not your thing, you could drive south to Bagno Vignoni and put your feet in the hot springs. And drive over to La Foce to see the estate where Iris Origo used to live - you cannot tour it (as far as I know), but you can park and walk around a bit.
Then head to Montalcino around 5pm when the shops open up and do your wine tasting. The stores in Montalcino sell these great olive wood cutting boards. I have several of them - they are great!
Then, as it is getting dark, head back home.
What do you think? There is lots to do in this area, but this would be a nice full day to show you some of the highlights. The only reason I suggest eliminating Montepulciano is that the couple of times we have been there it has been very crowded. You park outside the town walls, then walk up, up all the way through the town. I liked Pienza better because it was smaller and flat. The roads are busier around Montepulciano.