Definitely escape out of the city, by the seaside to some decent beaches supplemented by a nice collection of small stores, cafes and restaurants. Within 2-3 hours of Sydney. I disagree with people suggesting stay in Sydney - it is far too busy, too concrete and yes Sydney is just too busy.
Firstly to manage expectations, the urban sprawl of Sydney goes on and on in all directions, what we need to locate is that great little location, most probably just next to a main town.
First option is, as mentioned above, Gerrigong. South of Sydney and just south of the main town of Kiama, this is a prosperous middle class small town with all the required services. It is one of the few coastal towns on the railway, with an hourly service (connecting in Kiama) from Sydney, ca. $AU9 one way (yes $9! super cheap). Nearby one stop away is the town of Berry - which makes a great place for wandering about and having lunch. If you can hire a bike to explore nearby and / or take on the train that opens up a few more day trips otherwise lovely beach. Kiama is also worth considering, historic pretty town with a services and accommodation. My aunt lives there and I visit here often enough. My only caution would be any later than end of March and it would be too cold for my fussiness. For an NYCer maybe ok though.
Going north, for 2-3 hours this the Central Coast - lots of sprawl but there are a few hamlets where easy days on the beach are guaranteed. This is where I was born and grew up. It is a combination of train to Gosford and from there the local bus to the end destination. The buses and trains are timed to connect so it's quite easy, obtain your Opal card in Sydney and it is the same ticket system. I was there last weekend and was so relaxed I wanted to stay all week. There are two main tourist centres - Terrigal as mentioned above, and The Entrance.Once you are in either town you don't really need to go anywhere except to the sand. For extra quiet I would suggest rather than staying in the main centre stay just either side. So for The Entrance stay at North Entrance (Dunleith Caravan Park has cabins) easy walk over the bridge into town for all your needs, kilometres of endless beach, coffee shop, direct bus from Gosford station. Slightly further north is the new Pullman at Magenta, right on the beach in the middle of a national park (bus to The Entrance and then a taxi for the final 5 kms). Or stay at Blue Bay / Toowoon Bay - both have caravan parks (Blue Bay caravan park is not directly on the water), or perhaps try for a small hotel / holiday flat. If feeling very flush there is Kims Camp starting at $AU1200 / night. Ocean Front is a well located small hotel. I stayed at in the centre at The Entrance Hotel, a modernised 1920s hotel with amazing views, rooms are $A120/night. The Entrance is a mid market location and on sunny weekends the main town is busy with families. Down on the beaches it's lovely especially Blue Bay. Given The Entrance sits on a peninsula it feels isolated. Water stays warm until late April.
Terrigal is busy, slightly more upmarket. It is not quiet with a Crown Plaza in the middle of their high street. You could however look just nearby at Avoca (which is different to North Avoca). Small set of shops with all you need, lovely long beach.
Yes 2-3 hours is still, alas urban sprawl in Sydney. The alternative is to take a flight up to Port Macquarie, Coffs Harbour, Grafton (for Yamba) or Ballina / Byron Bay but then you would need a car which becomes expensive.