On the volcano by boat
You can take a day trip to the volcano by boat from Gialos (the οld port), Athinios (the new port) or Ammoudi, near Oia.
From the port the boat takes you first to Nea Kameni (the volcano) where you can walk to the rim of the volcanic crater; it is not an easy hike, especially in the hot summer days and if you are not physically fit.
The path goes uphill all the way, and there is nothing but grey and black rocks all around. However, the magnificent view towards the Caldera and the villages is definitely worth it all!
The boat will then take you to Palia Kameni and the Thermal Springs. The captain will drop anchor in the shallows, where you can take a swim. The waters are yellowish and five degrees hotter than the rest of the sea, and the smell of sulphur is overpowering. Soon after that, you will arrive at the picturesque islet of Therassia.
In the afternoon, the boat will sail along the impressive Caldera passing below Oia, and then will go back.