Monsummano Terme is a small Tuscan spa town that feels far less overrun than its larger cousin Montecatini Terme just a few kilometres away. The main attraction is the Grotta Giusti Terme, a thermal cave spa built inside a natural cavern lined with stalactites and stalagmites, where the air temperature reaches 36°C in the deepest chambers. You receive a robe at the entrance and are advised to spend no more than an hour inside, after which you can bathe in the outdoor thermal pool.
The experience is unusual and genuinely restorative — not the kind of slick resort spa you'd find elsewhere, but something more elemental. The cave is part of a luxury hotel complex, though day visitors are welcome. Weekdays are quieter and cheaper (around 14 euros for the grotto and pool combined); weekends draw bigger crowds.
Getting here is easiest by car — take the autostrada exit for Montecatini Terme and follow signs. The train station is about 4 km away, so you'd need a taxi if arriving by rail.