Pack yourself an awesome picnic full of cheese nibbles and maybe some bubbly and head to centennial park for a beautiful picnic anywhere on the grounds. Next to one of the big ponds or underneath a beautiful tree.
Take the 333, 380 bus from the city and get dropped off on oxford st and walk down into the park. Alternatively get the train to Bondi Junction and its about a 20min walk to the park
its best to go when the weather is nice. pack a lunch or if you go during summer you can go early afternoon or evening and watch the sunset.