You can spend one morning for a mini-cruise to Therasia and to Kammeni, in order to visit the Volcano and to swim to Zesta Nera (warm water). You can also visit all the famous beaches of the island, during this cruise.
The island is famous about the wines. At Santorini one can find plenty traditional wineries, which are open to visit. There one can learn everything on the local vineyard varieties, on the special way of cultivation and the procedure of winemaking and of course to taste the special wines.
The prehistoric settlement, one of the most important prehistoric settlements in the Cyclades islands, is a very interesting destination and you have the opportunity to follow some guided tours, daily organized by the archaeologists.
Moreover, you have to visit the museum of Prehistoric Thira, at Fyra, one of the most important museums in Greece, where there are findings from the excavations at the area of Akrotiri, as well as the Archaeological Museum.
Another monument you have to visit is the Monastery of Prophet Elias, which is over the area of Pyrgos, one of the most beautiful settlements of the island. It is built at 1700, at the highest point of the island, and it is the ideal place to enjoy the Rise of the Sun.
You can also enjoy a lot of other activities like excursions that you can explore the old villages, hiking the traditional paths, scuba diving, horse riding and extreme sports.