Nisyros leads the list of the best Greek islands according to National Geographic

1. Nisyros: Walk on a volcano

According to Greek mythology, it was Poseidon who seized a chunk of Kos and hurled it on top of a fiery Titan called Polyvotis — creating the volcanic island of Nisyros, with its enormous, sulphurous caldera. The volcano is active, but travellers can still walk across its warm crust — it last erupted in 1888 and is carefully monitored. It can be visited on a day trip from nearby Kos, but it’s worth sticking around to explore the island’s little capital, Mandraki, with its colourful houses and Byzantine churches; the nearby thermal spring at Loutra; and the lofty village of Emporios, with its ruined Byzantine castle, natural cave sauna and mesmerising views over the caldera.

https://www.nationalgeographic.com/travel/article/best-greek-islands-visit-this-year