grab some dinner (large selection from nearby restaurants), sit on Bondi beach and watch the sunset.
make sure however you bring a rug as sand flavoured food isn't very nice ;)
As the sun goes down, the normally very populated beach becomes very quiet and the sound of the waves crashing creates an amazing atmospheric feeling.

my recommendation is not going on a weekend, as music from the nightclubs just on the above pier over sounds the waves which doesn't create the atmosphere.