You could try King's Beach, it is on Brenton Point. It is not a beach, it is an old boat ramp and one side is a sheltered cove. A lot of dive shops use King's Beach for first open water dives due to the normally calm water. Just down the road is Brenton Point Park, there are large open fields and restrooms. It is a real beautiful area to sit out and have a picnic and fly a kite with the kids.