Nisyros is a small volcanic island a short ferry ride away from Kos. According to a legend, it was created during the war between gods and giants. It's a great option for a day trip - unless you get seasick easily, I would highly recommend it. An organised tour is a good option, as it will take you up to the spectacular volcano before free time to explore Mandraki, the capital of Nisyros.
Nisyros volcano is still considered to be active, as a matter of fact, the most active of the Cos caldera, and you certainly get that feeling when walking around in the crater - there is steam and scalding hot liquid bubbling along in the holes in the ground. My best advice is not to try and touch the hot areas - it will hurt! The most recent explosion took place in 1888.
With its alien-looking landscape, it certainly is a different kind of sight to visit and a very enjoyable experience.
Mandraki is the capital of Nisyros and the docking poing of the ferries. It is a lovely small town, very photogenic, full of white houses with wooden balconies and blue doors and shutters. If you like wandering around through quaint narrow streets, you will love it there. There are also seafront tavernas, where you can have a very pleasant lunch.