Dolomites, Lake Garda, Tuscany

Siena is about one hour away from the Val d'Orcia by car. Visiting Siena would take a good portion of a day.
Pienza can be seen in a couple of hours or so, including the time to walk from the parking lot to the city center.
Montepulciano is slightly bigger than Pienza, but can be seen in a few hours.
Thermal Baths (Bagno Vignoni or San Filippo or Chianciano Terme are the closest) depends on how much time you'd like to spend soaking in the water. A few hours maybe?
If you have a rental car you can see several towns in 4 days. Without a car then it is more problematic because bus service is limited (and almost non existent on weekends, and Oct 9-11 is a weekend).
A car is recommended also for the Dolomites. You can do without if you stay in Ortisei which has good bus service, but if you want to see Cortina or Lago di Braies (both about 1.5 to 2 hour drive from Ortisei) then a car is useful to be able to see them without changing your Ortisei base.
For your specific trip, since you are headed to largely more rural/mountain regions, I'd definitely rent a car.
The drive from Val d'Orcia to the Dolomites is a boring 6-7 hour drive plus pit stops (mostly on 4 to 6 lane freeways), but if you are in America, I'm sure you are used to that type of long and boring freeway drive. You can also break it into a two day drive, if 6 hour at the wheel is too long for your taste (I've driven from the SF Bay Area to L.A. many times, which is a similar distance, so it wouldn't bother me too much).

My husband and I are putting together a last minute trip to the Dolomites in early September. I am curious what makes Ortesei a better base than Kastelruth as recommended in the Rick Steve's guide? It looks like a different set of cable car systems and trails. I don't see many lift options from Kastelruth itself. I wonder why Rick recommends this as a good base?

So do I! I've stayed in Ortisei twice and there are so many areas that are easy to access from there.

Many of the lifts in Val Gardena close in late September or early October so check the schedule and try to time your stay in Ortisei accordingly.

It’s a pretty village, but not the only one and there are many just as pretty. Rick Steves likes Kastelruth, what can you do? I guess the first time he went to the Dolomites he went there and fell in love with it, and our first love is hard to forget. Many of my friends in Italy have never heard of it, including some who to the Dolomites to ski often. But many of my friends have not heard of Civita di Bagnoregio either. To each his own, but that shows that Italy is full of gorgeous places and you don’t need to follow every place that Rick Steves goes to. There are many more spots that others consider more important. Personally I wouldn’t go to Kastelruth without a car. Ortisei is definitely a better base if you plan on using it as a base and relying on buses.