This island, with a year round population of 1000, is both very green and very rocky, and stands out among the Dodecanese for its volcano, which is a main tourist attraction, but by no means its best.
There's also a very nice and authentic Greek port, good food, monasteries, a castle, nice villages, a hot mineral water spa (though requiring a doctor's note), good walking, black sand beaches (at Pali), and reddish sand beaches) at Lyes and Pakhi Ammos).
If you really need white sand beaches, there are those too, but you have to go to the nearby islets of Yiali and Aghios Andonios for those. There's even a hostel in the village that sits up above the volcano, Nikea.
Nisyros was settled during Mycenaean times, attested to by the massive walls at Paleokastro (Old Castle). Like most volcanic islands, Nisyros has fertile soil in places, with citrus, olive, fig and almond trees, as well as the gypsum and pumice quarried here, which has been a source of income for the island.
Though there is some low key tourism, lack of water keeps the worst of it at bay. The port and capital of Mandhraki looks pretty touristic at first but away from the water is like an old Greek village, though with the anomaly of two storey houses with painted shutters and wooden balconies, and some of the houses themselves painted in bright colors. Along with some cobbled lanes, some with pebble mosaics, a fine town square, a community orchard, a monastery, and some good old fashioned tavernas with local specialties,
Mandhraki is an altogether pleasant and interesting harbor town to visit and stay in, with some affordable, simple accommodation. The 14th century Knights' Castle and monastery overlook the town and are among the main sights, the 15th century monastery sheltered by the castle walls, whose church has some fine icons which much gold and silver in them. The monastery has a few rooms for guests.
The 7th century Doric castle (Paleokastro), is on the acropolis of ancient Nisyros, about 20 minutes on foot from the port, its walls cut from volcanic rock, still at their original height -a truly spectacular monument.
The island also has some picturesque fishing villages (Pali ) which is non touristic, and near a chapel built in an old Roman bath house. Lies (or Lyes) cove has a sand and pebble beach with a snack bar.
The volcano is in the center of the island, and though dormant, is still bubbling and emitting sulphurous fumes, having erupted as recently as 1933. The views of its craters from the two villages (Nikea and Emborio) on its rim are quite remarkable. Emborio has a ruined fortress, and a tiny population, swollen lately by both Athenians and foreigners who are restoring the old houses. A public steam bath in a grotto is located in the village; an old cobbled path behind a taverna leads down to just above the craters. Both Emborio and Nikea boasts fine views down into the craters but Nikea also has some out to Tilos island, and some lovely squares, one with a pebble mosaic, and several tavernas. A path from Nikea down into the volcano leads past a frescoed monastery, also on the crater rim, where an annual festival is held on September 25th.
There are five craters in the volcano, the largest and most visited, Stefanos by name, which is 80 feet deep and 1150 feet across. Solid shoes are recommended if you don't want to burn your feet on the hot surfaces. There are many other walking options on the islands, with a good free topographical map available at the tourist offices, one to Moni Evangelistras Monastery, with three different trails leading there from Mandhraki. A three hour walk from Nikea to Mandhraki is also a nice walk, with another monastery (Stavros) on the way, which hosts a festival on the 13th and 14th of September, with lodging for guests for that event.