New Zealand is really beautiful! But you will really need to be able to drive around, unless u r planning to stay in one city (most of the tours will pick you up from the hotel). There r buses to diff towns, but not much within the city or to outskirts.
About campervans, though they look really cool, I heard from a friend staying there that they r not nice to drive (something dragging at the back).
To me, the ultimate highlight was kayaking in the Abel Tasman national park where a baby seal swam right under my kayak! Loved all my wildlife experiences there.