Best thing to do in NZ is try and do as much as you can. If you have seven weeks, that's great. I only had four and I jsut ran myself ragged doing everything and I'm so glad I did! Definites are Kaikoura, Wellington, Oamaru, Queenstown, Hokitika, Milford Sound, Tongariro Crossing.
Definitely the most arduous thing I've ever done was the Tongariro Corssing in freezing windy weather but it was so worth it. Enjoy is and do as much as you can in seven weeks.