We visited Sifnos in September a week after the high season. Vasiliki and her sister Maria are perfect hosts. They gave us all sorts of tips. Where to go for a hike. Where to eat and always had plenty of tips what to do. They made us breakfast in the morning and a wine in the afternoon. The location is breath taking. We rented a beach front family house: Ambelli. The apartment had a separate bedroom, couple of additional beds and a kitchen. Great terrace and deck with direct view on the sea. They have a large swimming pool area where you can chill. They offer yoga lessons in the evening which was a great experience too.
The island is easy to go around. The city of Artemonas was a great hidden pearl with it's narrow streets and beautiful houses. It's a quiet island with lot's of things to do. A real recommendation.
One tip. Just take a wind jacket to the island