If you havent been to Sydney then I highly suggest doing so.
The City itself is amazing , but I am more of an adventurous get out and explore the walks/beaches type and Sydney definatly has alot to offer !
I have only two tips... visit in summer and visit as many beaches as you can.
Today I visted Watsons Bay (below)
While it is a great spot it doesnt have a very big beach and the water is fairly shallow so I walked 5mins through some quaint little streets till I stumbled across the gem that is Camp Cove (below). I would give this place a solid 8/10 amazing golden sand, not too crowded, deep calm water and very sheltered from the wind.
If you want a break from soaking up the sun you can do the leisurley walk to Hornby Lighthouse where you will get views of the entrance to the harbour. I walked this taking pictures on the way and it roughly took 30 mins to do the loop.
Hope you enjoy !