Depends what sort of things you like to see; but there are many. For adrenalin sports, Queenstown (bungees, white water and so on). For thermal activity - geysers and hot pools - Rotorua. Towns, Napier (art deco buildings put up after an eqrthquaske int he 30s) and Wellington (beautiful harbour) are my favourites. For trekking, just about anywhere. Scenery: Bay of Islands, Southern Alps, but again just about anywhere. Whales: Kaikoura. Wine: Marlborough.
The trickiest thing is working out what to see then building an itinerary round it; it's a long, thin country on at least two islands but without all that many roads (compared to Britain). A friend of mine has just returned, having flown into Auckland and out of Christchurch, which is a good way of doing it.
http://www.newzealand.com/travel/UK-Ireland/