We have visited some of the hot springs in Tuscany. Most of my information is old and is over on SlowTrav. I am updating it now for Slow Europe, but don't have anything posted yet.
Saturnia (southern Tuscany, near Pitigliano). These are the most famous of the Tuscany hot springs. You can sit in a sort of waterfall of hot water. We did this (years ago). It is south of town, just off the main road. There is a parking area. Nearby is the Saturnia spa. From what I have read it is nice, not the medical kind. Saturnia is a lovely town with good restaurants and a bit of Roman Road and an arch in town. This part of Tuscany is very interesting. Pitigliano is fabulous. And there are Etruscan Pathways - paths dug into the rock that they used to travel from town to town - nearby. I am making a day trip for them (soon). Let me know if you decide to go there and I can give you more info.
San Casciano dei Bagni (southern Tuscany, near Cetona). The spa here is good. Friends of ours went to it. I like the outdoor natural spring, but it is small. From the center of town, you walk down stairs to the river below. They made two open air pools. They are not deep.
Bagno Vignoni (Val d'Orcia). Lovely little town. The hot water runs out from town across a field and then down a cliff. You can sit beside the narrow channel and bathe your feet. We revisited this one last year. The pool in the center of town is from medieval times and is not in use. The town has several spas and there was a thread about them recently. I will look for it. We have not been to them, but I know several people who have and who recommend them.
There are supposed to be open air hot springs near Viterbo, but I have not been to them. There are also some in the woods in southern Tuscany near San Filipi something-or-other. I will look it up. We visited those years ago, but they were not that great.
Gloria from Pisa has written about hot springs in her area on her blog. There are also some up in northern Tuscany.
If it were me, I would go to the outdoor hot springs in Saturnia and also to a spa in Bagno Vignoni. I think a short stay there would be very nice in winter!!