Nisyros is a volcanic island roughly 25 nautical miles south of the island of Kos. There are daily trips by ferry out of Kardamena that will take you to the main port of Mandraki in about an hour.
The main point of interest
is of course the dormant volcano which you can reach by bus, and that is usually the first thing people do. The bus leaves you a few hundred meters from the main crater called Stefanos which you can go down to by way of a little path.
You should take care though because the ground can get pretty hot and light shoes (flip flops for example) might not be that resistant to the overbearing heat. Inside the crater you will probably hear the water hissing as it reaches boiling point and will not fail to distinguish the acrid smell of sulfur.
There is a small cantina next to the bus terminal where you can grab some water or other refreshments when you get back from your adventure.
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Once you get back to Mandraki there is a Historical and Archeological Museum that you can visit and you should definitely climb up the pretty alleys to the Monastery of Lady Spyliani which is right on the edge of the castle built by the Knights in 1315.
The town is rather small and can be visited in less than half an hour but it is well worth exploring as it has blended with the terrain in a very harmonious way. Its whitewashed traditional houses right on the waterfront are quite striking, and the various little tavernas and cafés and bars are really appealing, especially after the strenuous trip across the mountains to the volcano and back. It is the ideal place to sit back, relax, and have a refreshing drink, especially in summer when the heat can become somewhat unbearable.
There are quite a few shops available, and actually some art shops with some interesting or unusual things. You can try out the cinnamon or almond-based drinks, which are a specialty of the broader region but are rarely commercialized on the other islands.
There are a number of smaller villages on the island that you can also visit, but you will have to get there on foot or by car (either by renting a car-readily available at the port or by taxi). There is a little cobble (volcanic in origin and mostly black in color) beach on the other side of town where you can go for a swim if it really gets too hot.