There are several natural thermal springs in the Tuscany region, some of them quite famous and historical. An American family spending a year here in Florence had a chance to hit the road and visit several recently - here is their short take on five of them:
1. Terme di Saturnia: cascading waterfall pools, body temp. Out of a dream!
2. San Casciano dei Bagni: smallish, hot tub temps, relatively unknown.
3. Terme di Sorano: private. Included actual pool where boys used goggles; just above body temp. (can also pay for massage, other services, etc.). Quite busy.
4. Bagni di San Filippo: calcium deposits look like huge snow drift. Really hot water spilling over. Runs into river. Hiked a path to find it; boys LOVED this one!
5. Bagni di Petriolo: hot hot wonderfully hot. Smallish hot-tub like pools to populate with friends. runs into cool river---nice contrast. Busiest of them all (at least the day we visited). Bring wine!
The last one we visited was way too long ago in Bagno Vignoni.
You can read more about their trip and the baths on their blog, Family Frolics.