Palm Beach is the most northerly of the Sydney beaches. While being quite a way from the city (from 45 minutes - 1 hour), the trip there is well worth it, if you can take the whole day. The beach itself is about 2-3 km long, with lovely clean sand all the way. The commercial section is only on the southern tip of the beach, parking is available to about halfway, and the remaining half (including a sizeable sand dune at the end) is almost always sparsely populated, and so very peaceful.
The water is probably the cleanest in Sydney, and there is often a decent swell for those who want to surf/body surf.
If you get bored of lying on the beach, you can do a walk up onto Barrenjoey Head and see the lighthouse, as well as look across Broken Bay to Brisbane Water and the central coast.
A much nicer beach than the way-too-commercial Bondi and Manly.