The far north in NZ. Can do Whale/dolphin watching etc and really good beaches. Fly into auckland, hire car, can get a whole house on bookabach. Or Rotorua has lots of stuff to do and see. Central Otago is lovely at that time of year, Queenstown and Wanaka have things to do and Cromwell is a relaxing little town if you are after serenity.

But Hamilton Island is a good idea too, or something else in your own backyard that you have never got around to doing. Gold coast hinterland is nice. If you are just wanting a beach type of relax then you can save airfares and go up to Noosa (or just out of for more quiet at that time of year). The Eumundi markets are quite cool. I rode a camel there. P.S, don't ride a camel several weeks after giving birth.