€18 each at an excursion office in Firostefani to take a trip across to the volcano island the next afternoon. (We also had to pay an additional €2 once on the island.) Wear comfortable shoes, grab a bottle of water and slap on some sunscreen and you'll be good to explore.
Thirasía, the other inhabited island in Santorini, and Palea Kameni. (Santorini is an archiplegao which consists of Santorini (or Thira as it's officially known), Thirasía, Nea Kameni, Palea Kameni, plus another uninhabited island: Aspronisi.
The white buildings on top of the volcanic caldera on the mainland could almost be mistaken for snow-covered mountains! Oia is to the left of the picture, whilst you can see the village of Imerovigli to the right.
°C - it certainly felt warmer than normal sea temperatures, but not *that* hot in the bit where we were swimming. Be warned, if you're wearing light swimwear, you will get sulphur stains on them.