Rotorua on North Island is an area of vast geothermal activity, and is also considered to be the home of the Maori people; Rotorua provides a unique chance to witness the power of the earth as well as immersing yourself in the local culture.
The city of Christchurch on South Island has a very English feel to it, it has a real café culture vibe, and is a good entry point into the Southern Alps - which are a great place to experience great mountain sports including skiing.
Queenstown has become the centre of an entire tourist industry that has grown up out of the Lord of The Rings Movies, and close to the town in Fiordland, you can visit many of the locations used in the films including Macetown, which doubled as Isengard, Skippers Canyon that played the part of the Misty Mountains, and even the aptly named paradise, which was the site of the fabled Lothlorien.