Which places in Tuscany are little-known hidden gems?

I would especially recommend the trio Sovana, Sorano and Pitigliano, in southern Maremma: the villages are known locally, but often remain off the classic Florence–Siena itineraries. Their appeal lies in the alleys carved into the tuff and the vie cave, ancient sunken paths between the walls. You can walk some of them around Pitigliano and Sovana; plan on good shoes, water and several hours. In midsummer, the heat makes walking less pleasant. A car helps a lot for getting from one village to another; a regional loop between these towns is about 80 km.

For a quieter atmosphere, add Montemerano, a small stone village near Saturnia, or Bagno Vignoni, in Val d'Orcia: its central square is a large thermal water basin, but you can't bathe there. The Parco dei Mulini, en contrebas, vaut une courte promenade. Ce n’est pas un village à visiter pendant une journée entière : combine-le plutôt avec San Quirico d’Orcia, à quelques kilomètres. Pour éviter les groupes, vise le printemps ou l’automne, et arrive tôt en été. Ces endroits sont moins pratiques sans voiture, et les petits restaurants ou commerces peuvent avoir des horaires réduits hors saison.