Travel to Tuscany with teenaged daughters in March - too many stops?

I think you are wise to stick with a somewhat limited geography and not go further south. To me, the place to use your extra night would have been Florence, but it looks like you are sandwiched by two other firm reservations so that is not likely possible. I would then add the night to Montepulciano or Lucca, since you'll have a car. From Lucca you can go to Pisa, the beach at Viareggio (or another beach), Montecatini Terme where you can take the waters, Pisa, and more.

From Montepulciano you have so many hill towns you can visit.

You say "We will have a car during this period." Which period, exactly? You do not want nor need a car in Florence nor Siena, and really you can do so much out of Lucca without a car.

I have only been to Florence, Lucca and Pisa on your list but, if I planned visiting them plus Siena and Montepulciano and had a car, I would stay the whole time in Siena or somewhere nearby. The other destinations are easily reachable by car for day trips from such a location and, that way, you get to stay in one place instead of packing, unpacking, checking in, checking out......

Alternatively, stay in just two places, one between Siena and Monte, the other in Florence/Lucca.

Since your plans are already set, I don't think any advice you will receive contrary to your plans is helpful. That said, I think you have not allotted enough time to either Venice or Florence given your stated interests. 2 nights in Florence only gives you one day which is definitely not sufficient to see and experience all Florence has to offer. As you are really only asking what to do with your one night that hasn't been booked, Volterra would be a good choice.

I hope that is not your sequence since Montepulciano is very close to Siena.
Perhaps u could visit also Assisi while u are in Tuscany area it is very special.
Your daughters may enjoy visiting the etruscan cave sight and museum in Chiusi

Your girls will love this place...

https://www.wanderingitaly.com/blog/article/1066/bagni-san-filippo

45-50 minutes south of Montepulciano, the "Perfect" Spa day.

Thanks all for your advice thus far. We will have a vehicle when we depart from Florence until we arrive in Siena.

We have visited Florence previously and do not expect to see the whole city, nor do we want to spend time in larger centres. In Siena we want to experience the town by foot and relax into the experience, so not looking for day trips over the weekend and there would be no overlap with other cities/towns in the area.

While it may look like I’m only looking for advice for the one night I’m quite willing to cancel our booking in Lucca if it made sense, and seeking thoughts re what to do for the three days between Montepulciano and Siena.

Apologies if this wasn’t as clear as it could have been in my original post.

Appreciate the link for the spa experience!

Thanks.