Sifnos without a car is possible, but it needs rhythm
Can you have a good time on Sifnos without a car? You can. As long as you do not want to be at three different ends of the island every day, swim wherever you saw in a video, go back for a shower, leave again for sunset and then search for a table in another village. In that case, no, you will get tired, and the plan will be to blame, not Sifnos.
If you do not have a vehicle, stay somewhere central or in a place that has life within walking distance. Apollonia is practical because it has food, coffee, shops and good bus connections to the whole island. Artemonas is quieter and very beautiful, also with good bus connections, but with more uphill walking. Kamares is convenient for arrival and departure, with good facilities, many restaurants, a beautiful beach and coastline, life from early in the morning but not late at night. The seaside villages are also wonderful, with fewer bus connections, and they definitely need better planning.
The secret is to choose one area per day. One day for the southern beaches, one for the central villages, one for the north. Check the timetables, leave time between journeys and remember that short distances on the map do not always mean “I can go whenever I want”.
Without a car, Sifnos forces you to slow down. And between us, that is not a bad thing.