Take the Trans-Alpine train across the Southern Alps, and back if you like! On the west coast of SI try to get to the Pancake Rocks at Punakaiki, down to Franz Josef/Fox glacier and if funds permit, a helicopter ride over Mount Cook/Mount Tasman and the glaciers. Around Blenheim visit the vineyards.
North Island..would agree with Epiphany about his suggestions, but would add Orakei Korako,( between Taupo and Rotorua). It`s off the beaten track, but excellent for volcanic features and far quieter than Rotorua.
If you`re in to Art Deco, then you`ve got to go to Napier, rebuilt after an earthquake in the 30s. Also the wine areas of the east coast of NI.
I could go on, but I`d still be here tomorrow! You can tell we had a brilliant time over there. So much depends on what you are interested in.