At only 2-3 hours drive of Sydney, there is a gem that many Sydney-siders actually never explored: the Stockton Sand Dunes next to Port Stephens. It is quite a trek to get there as you have to find the right entrances, which you can along the dunes but there is also the option of discovering part of the dunes and an amazing beach by Anna Bay.
You can also drive on the beach by the dunes given you have a 4WD as well as purchased a permit from one of the petrol station before the dunes. Of course, you also have to be quite at ease with driving on sand and your tires should be a bit softer.
The must is to go during winter as you'll be able to see the most beautiful stars and milky way ever. We did with some friends during the 2016 winter and this was a truly magical moment.
I highly recommend you check the Stockton Sand Dunes and most of all bring your camera as it's quite another world!