Megalo Livadi is probably the best beach on Serifos, where personally I didn't find great beaches. Megalo Livadi is just fifteen minutes drive from Chora. There is a bus going there but itineraries are quite rare. Also the driver may be a bit late, as there is traffic in Chora sometimes. Otherwise, the beach is clean and particularly quiet. There is much tree shade around, so you do not need to bring umbrellas of your own. Just a few supplies, like water or light food. If you don't, you can have a meal at the taverns there.