Have you ever gone to have a bath at open air in the middle of February in Tuscany? I did.
Just go to the Terme di Saturnia. There you can choose whether to go to the pools at the source and make the input (daytime only), or go a little further down the waterfall spa where the ditch of hot water that comes from the spa, after a short distance down, between fields, arrives to an hot waterfall close to a mill (Cascate del Gorello).
Here the entrance is free. The location is beautiful, one of the most photographed of the Tuscan area. The sulfur water has created calcareous concretions that make fair the whole area of the waterfall as festively decorated for sultans and concubines. There are several natural pools where the water flows constantly at a temperature of more than 30 degrees (38° to the source).
At the level hygienic you know... there are two very different situations: at the source, you have a steadily maintained and controlled pool area with services; at the water falls is the area is strictly rural, missing of any services.
... different locations but surely unique in both cases.
In summer, on special occasions, the pools are open also during the night, with candles and poolside live music. A treat as true romantics.