I don't know that many of the towns are "off the radar" -- but last Fall we stayed 2 weeks in Umbria and very much enjoyed Cortona, Spoleto, Spello, Montefalco, Todi and Perugia among other places. You can check my trip report for some stand-out suggestions http://www.fodors.com/community/europe/heavenly-2-week-retreat-in-umbria.cfm

Not exactly off the radar, but add Gubbio to uhoh_busted's list. Same with my TR-a couple of places you didn't mention above:

http://www.fodors.com/community/europe/perugia-and-rome-by-train-and-bus.cfm

Check out my trip report from last year---click on my name---to find the 3 small villages that we explored for lunch during our week there last May. Especially La Porta in Montichiello.

San Quirico d'Orcia
Castiglione d'Orcia
Rocca d'Orcia (Cantina Il Borgo)
Radicofani
Monticchielo (Taverna di Moranda)
Montefollonico (La Chiusa- expensive), Osteria La Botte Piena- inexpensive- try the baked cheese dish- fantastic. Sit outside)
Castelmuzio
Montisi
Petroio
Lucignano d'Asso (a small hamlet with a meat and cheese shop that serves a nice lunch platter- very casual)
Sarteano (La Giara- not sure if it's still open- not pretty, but the food is outstanding and inexpensive)
Trequanda

Further south-

Sorano (Cantina Ottava Rima- a snack/wine cellar serving casual, fresh food, cheeses and wine from the area. Fantastic- one of my favorite places to visit. It is located in a cave right in town and owned by an artist- go! Open on weekends only)
Pitigliano
Roccalbegna
Santa Fiora
Piancastagnaio

I'll also be in the area for 2 weeks in late September. If you enjoy easy hiking, let me know and I can provide some suggestions.

Todi, Norcia might fit your bill.

Please understand that it is actually rather difficult to find a town in that area that isn't of interest, so you can simply point your car in any direction and press the gas pedal, and stop at the first town you see that you've never heard of. You'll be amazed nobody talks about this wonderful place.

If you come to a big-time tourist board like this, you are mainly going to get recommendations to go to places that are already quite popular with tourists. Todi was voted "best place to go in Umbria" almost 10 years ago by one travel magazine. San Quriico d'Orcia is right there is Fodor's guidebook to Italy. Cortona was the setting for "Under The Tuscan Sun." Pitigliano does a robust bus tour business -- and if you are staying in the val d'Orcia, Gubbio is way too far to drive for a day trip, and I would take Spoleto off the list too unless your idea of lovely Tuscan stay is spending a lot of time on the autostrade going to a "destiantion" rather than exploring the quirky back roads.

I highly encourage you to go off on your own into Western Umbria if you simply want to go exploring.

If you would like to seriously learn something about the history of the region you are visiting, then hiding in plain sight, right at your door step, is one of Italy's finest small museums of Etruscan excavation in Chiusi, a sweet untouristed hilltown Fodor's posters (with two exception) have no clue about. You can also visit the absolutely charming flat town of Buonconvento and visit its museum about the lives of the sharecroppers who created the Tuscan landscape over centuries, and how their descendants survived starvation to become wealthy by turning that same landscape into a playground destination for affluent tourists, too many of whom stampede around the hilltowns shopping and snapping up picture postcards and never give 5 minutes to learning anything about the historic reality of where they are.

Yes, try to avoid anyplace that has proven to be of interest to others. But do not miss the feeeble Etruscan museum in Chiusi---that should take at least :30. Have fun !

zoecat, that's an amazing list!

Here's a link for things to do in Spoleto. The Hotel Clitunno had an A+ restaurant when we were there. Again, as I qualified my statement above, although it's not exactly off the radar, it's still a really great place:

http://www.italianvisits.com/umbria/spoleto/

Do check the michelin guide ( http://www.viamichelin.com/ ) for driving distances. How esoteric do you wish to be on your trip?

In western Umbria from your base-

Citta Della Pieve (a lovely town located 15 munites above Chiusi. The Saturday market brings out all the locals in the area)

Castiglione del Lago (right on the lake. If it's a warm and sunny day, take the public boat across the lake to Isola Maggiore and take a walk around the island)

All of the towns around Lago Trasimeno are worthy of a look-see. And in summer there are some fabulous concerts inside the lit-up castle ruins on top of the hill in Magione.

In southern Tuscany there are myriad hot springs, some of them made into fancy spas, some of them free if you know where to hike, and some of them simple, unpretentious places. Get a detailed map and take off and get lost and find a few (if you like that sort of thing).

StCirq's post reminded me--there is a lace museum on one of the islands on Lago Trsimeno.

Yes, it's on Isola Maggiore. And the old ladies still sit outside doing their lacework.

Magic. Everything was closed on the day we went there. One man sweeping his yard, cats in charge. Someone had a picture of a dog with a "beware of" sign. Perfectly still except for the sweeping sound. It was like an Ingmar Bergman movie.

Thanks to all of you for your posts. When I lived in Friuli back in 2000-2003, I hit many of the bigger, more well-know towns but this time, we will definitely visit many of your suggestions. As we are taking the train to Chiusi, we'll definitely visit the Etruscan museum. Again, many thanks to all.

The hill town of Amelia, located 30-40 minutes southeast of Orvieto, is worth a visit. The best pizza, using soy flour, can be found at Porcelli Tavern. There is a nice informal wine bar on the main street called l’Enoteca Punto Divino, which serves good food.

Chiusi was the very first town we stayed in back on our first visit to Italy in 1999 (I think). It was so charming and a great introduction to the little things that make the region great. The "old" centro is in the upper town. I think the rail station is in the lower town.

Cortona is definitely more touristed that it was when Under the Tuscan Sun was written, but it still has many charms. We lucked into a Classic Car rally the Sunday afternoon we visited. I think we spent about an hour and a half just agog at the wonderful cars, and enjoying talking to their owners and taking photos. It's the totally unplanned for events like that which make a trip unique and fun.