Definitely include the Blue Mountains (there's so many different things available to do and see in the Blue Mountains it's hard to say whereto start beyond suggesting the main tourist attractions around Katoomba).

If you have a few days up your sleeve and want to see a bit of country New South Wales but don't want to spend more than a day's travel from Sydney in getting to your destination, you could consider the area around Bathurst, Orange or Mudgee in the West, Kiama in the South or, if you would rather head North, somewhere on the Central Coast (maybe The Entrance or, if you just want to relax at a beach, Avoca).