If you have time in NZ, definitely get down to Queenstown or even Milford Sound on the south island... even getting the ferry from Wellington to Picton across the Cook Strait is quite beautiful and Kaikoura on the east coast of the south island (about 1.5hrs north or Christchurch) is amazing too (sealife, beach and mountains all in one hit)
if you don't have time, then the Coromandel Peninsula on the north island is beautiful, Rotorua is very interesting with its multi-coloured sulphur springs, and Mt Whakapapa (you'll laugh when you hear how its pronounced, "wh" = "ph") down the south end of the north island is meant to have some great walks. If you're into hiking NZ is the place to do it, they have an extensive trail hut system maintained by the Dept of Conservation and no cougars or bears to worry about....
Have a great time - oh yeah, February tends to be the hottest month...