i dont know which season is the best ( honestly ), because four season gives you four different experience. I can tell you i like all of it. I've been NZ 5 years and every year i travel at least twice between North and South Island.
For Summer which is December, It is the best time to North Island, for example, Bay of Plenty, Bay of Island,. I think no one want to go beautiful beach in winter, no way, too cold. I would say summer Christchurch is better then winter christchurch for my experience in past 4 years. If you like flower, trees, flora and fauna, Christchurch is nice. You enjoy beach and sunshine, Bay of Island, Bay of Plenty ( cool ).
For Winter which is about June til August. You can decided to go to South Island, especially Southern Lake and West Coast. Southern Lakes include Queenstown, Arrowtown, Wanaka... etc. West Coast include Fox Glacier, Franz Joseph. If you like Lake, snow mountain, ski, glacier. Winter is the best option for you.
Rotorua, Lake Taupo, Waitomo, Vocanic Area. Cool geothermal activities can be found in Rotorua and grow-worm can be found in Waitomo. Four season...
Mt Cook is the other good place. Four season give you four different experience. you can see snow Mountain in Summer as well.
Auckland, Wellington these major city, i think every tour will including in it, so i dont talk too much. Auckland you will may decide to go to Mission bay, Sky City, Harbour Bridge, Downtown Princess Wharf, Museum... etc I personally like Mission Bay, Downtown and Museum. I dont know why i can spend about 5 hours in such ( i expected to be ) boring place for this long and finally i feel interesting. Wellington, you may find Te Papa is a must go place. Mt Victoria give you full view of Wellington Harbour ( at night time, real cool )
more city description, place refer to these page. they chose me as the best answer (it have word limit) so i want to give you more detail in Souvenirs
related knowledge:
http://hk.knowledge.yahoo.com/question/?qid=7006102004394
http://hk.knowledge.yahoo.com/question/?qid=7006102002406
http://hk.knowledge.yahoo.com/question/?qid=7006101901616
Souvenirs:
I would say honey is the best options, there is lot of brands and lots of different type you can choose. most of the brand is from one factory in south island (certainly some of it is not many honey factory in NZ), dont worry about the brand, they all the same, but which type of honey. I personally dont like Manukau Honey, taste not sweet, I like Cream Clover.
Kiwifruit product like sweet and dry fruit is quite good. Greenstone (expensive) but worth to have a look and decide.
Manuka Honey hand cream, Lotion is good. I personally have it. Lanolin, if you want to buy it. Is good. but not sure about the brand. (dont buy cheap brand)
Sheepskin is the best in NZ, but really expensive. but quality is real good.
If you like sport, All Blacks products is you options as well, I am not sporty people, but i like this brand.
Remember in you trip, go to see Kiwi bird and Maori Shows, these are New Zealand logo and culture. Farm show is highly recommanded.
I have already tell you all the best things in New Zealand, you may find out more and tell other people after your trip.
2006-10-25 00:37:02 補充:
just refer to nice2cu000 most people dont like queenstown in summer, because it is too hot. if say jet boat, Taupo is better, still hot, but feel better. but i still suggest you to go queenstown, I think if you havent been queenstown, you havent been New Zealand.