Popular beaches in Sydney are Manly, Coogee and Bondi. All of them are nice beaches but sometimes they are just too crowded and too noisy. That could make you feel uncomfortable and you would not enjoy your day.
However, there are many other beaches that can perfectly suit your needs, even they don't get the publicity the others have. One of this beaches is Narrabeen Beach, on the Northern Beaches neighbourhood.
Getting there is easy- catch a bus at either Central or Wenyard Station, which goes through Palm Beach, and the bus will drop you just across the beach.
The beach is big and there is plenty of space for you to feel like you were almost alone in the world. If you are thinking going there with kids, there is also a rock pool which will be perfect for days with big waves.