Hi Martin,
Sydney has loads of different beaches, and different types of beaches too so I'm sure you'll find one you like.
Our favourite beach was Mona Vale Beach in the North of Sydney. It's got nice waves without being too rough and though it's popular it's large enough to never feel too crowded. The beach actually goes aropund a corner so it's like 2 beaches for the price of one. There's also Modus Operandi craft brewery not too far away for a drink and music afterwards.
On the south side we like Maroubra which is a long stretch of beach with some but not too rough waves. The area is also nice, there's a nice cafe there and the Grumpy Baker bakery up the hill.
If you want some waves to jump around in then try Coogee, also on the South side.
Our neighbour recommends Clovelly for small children (and anyone nervous in the water). The beach itself may not look fantastic but the water is protected making it practically wave-free.
We didn't like Bondi or Manly, both seemed over-rated, particularly Bondi which is too crowded and just...uninteresting. There's nothing wrong with it but it doesn't compare to the others. A good place for you might be to start with the Coastal Walk, you go past a number of beaches: https://www.sydney.com/things-to-do/nature-and-parks/walks/bondi-to-coogee-coastal-walk
