I spent a week in Kythnos at the beginning of September: no crowd, sun and excellent beaches. My favourite were Apokrousi and Potamia, but also Episkopi is worth a visit. Kolona was nice but too hot, no shadow and too many boats. It can be reached by car, but the road is quite difficult, some driving skill is required. The same for Potamia.
Most of the beaches on
Kythnos have gross, grey sand and many pebbles. Do not expect to find the white beaches of Kefalonia or Zakyntos. However, access to water is always easy, and you 'll find plenty of shadow under the trees that grow by the sea. There aren't renting facilities for umbrellas and chairs, so get your own.
Night life in Merihas is reasonably lively: lot of restaurants (my favourite was Gyalos) and a nice bar with internet connection and lovely home made deserts - Beggeles. The disco in Akrotiri looked very nice, but at 11.00 pm it was empty - apparently people don't show up before 1.00 a.m., and leave in the morning, around 6 a.m. (definitely off limit for an old lady like me).
For a nice accomodation I strongly reccommend Villa Elena in Martinakia : great view on the port and on the sunset, very confortable two-storey apartments, easy access to a small beach from the premises. To sum up, a very relaxing holiday in a place that won't disappoint visitors who look for a corner of authentic Greece.