New Zealand is a very beautiful place, relatively compact but with lots of variety and still a lot of true wilderness. That means you can squeeze a lot of different activities into a visit of only a few days.
The North Island is known for its beaches; Mount Maunganui is one of my favorites. It's also where Maori culture is centered, particularly around Rotorua, an area of geysers and hot springs that always smells like ass (sorry it doesn't hold a candle to Yellowstone). There are some cool cities - Auckland, Wellington, Napier - and quite a lot of heavily forested, mountainous land.
The South Island is known for big mountains (the "Southern Alps") with skiing opportunities in winter, and several fjords where the sea kayaking is amazing. It's also a very popular trekking destination (bring a rain jacket). And then there's Queenstown, host to (last time I was there) no less than 75 companies offering adrenalin sports experiences - from skydiving to river rafting to anything you can name.
If you want to check out the scenery, just watch the Lord of the Rings trilogy. The castles, fortresses, etc may have been CGI but the scenic backdrops were mostly genuine. Mordor was filmed on the South Island (some of it in the Queenstown area), while the Hobbit's Shire was on the North Island.