Must see's include Milford Sound, Doubtful Sound, Mt.Cook, the West Coast glaciers, the Banks Peninsula, Bay of Islands, Kauri forests. The thermal regions (there are many of them) on the North Island are all different in their own way, take the time to see them all. The cities are cities, the really good stuff is out in the country.
Look up Top10, and HAPNZ, holiday parks . . . excellent network of accommodation for campers, trailer homes, or self-catering cottages.