There are many things you can do on Ios island after the sunset. There are many restaurants, cocktail bars and nightclubs. Most of them are located in Chora, the capital of the island, among the narrow alleys.
You will not miss anything here. There are so many places and choices, that you will definitely return to your hotel satisfied. No matter which type of music you like, you will find something interesting. Another positive matter is that you can change places as many times you like, as everything is close to each other.