Theia is the leisure-focused thermal park in Chianciano, with large outdoor pools where bathing is treated more as a pleasure than a medical treatment. The outdoor setting gives it a different character from the indoor terme: you can float in warm water surrounded by Tuscan hills, which is difficult to argue with.
There are indoor facilities too, and a range of beauty and massage treatments. Day tickets give access to all the pools. It's busier in summer but the outdoor experience is arguably better in the shoulder months — spring and autumn — when the air is cool and the water is warm.