I'm going to New Zealand too! We'll probably be arriving just as you are leaving
Unless things have changed a lot in the last few years, it's almost impossible to go wrong. I've not been to North Island, which is busier, but on South Island even the touristy stuff is great.
* Whale watching in Kaikoura is good (plus swimming with dolphins and/or seals)
* Dart river jet boat
from Glenorchy is a good alternative to the Shotover thrill rides
* Heli onto a glacier is cool. We'll probably take a fixed wing
flight this time, and land high up on the glacier.
* The gold rush towns on the west coast are interesting, and the drive over Arthur's Pass is worthwhile.
* If you want to bungy jump then The Ledge and Kawarau Bridge in Queenstown can both be done without taking a whole day (unlike some of the bigger ones).
. . . and that's just a few highlights off the top of my head. Can't wait!