OP,
You lucky people I spent 5 weeks there in 99 and it was amazing.
Christ church itself is a bit dull but
unmissable IMHO would be:
Queenstown--a few days and depending on adrenalin levels you can bungee/parachute/hang glide/fly by wire/etc.... to your hearts content but its also a lovely town with scenery
There are boat trips(day trips) from there to a very famous(cant remember name maybe Milford) sound which if weather is good is amazing if weather is bad(more often than not)you will see not a thing
Franz Josef Glacier or Fox Glacier I did the former..you get to do a spot of ice walking and it can be short or long whichever you want but its great dont miss it
North of christchurch is great for whale watching expeditions
If up for it do a two/three day walk in one of the natural parks..some you need to be very fit for others not so much at all I recommend a 2/3 day walk (Its mostly flat so you dont have to be superman or wonderwoman)in Abel Tasman park which is at the north of the south island near nelson..they drop you by boat at your stop off point and collect you 2/3 days later, you sleep in large huts(so you meet lots of others) so you get supplied with food and gas and camping cooker which you carry with you..really fabulous
I think you can get away with booking most things when you get there(gives you far more flexibitity too) you are going there in spring time so would not imagine too bad driving is fine but also there are great bus tours some aimed at teenage gap year types others for more mature(i.e 25 to 95) I did a magic bus tour (not sure if they still exist but it was very good) also hostels are great there so dont be afraid to stay in them they are excellent ways of meeting others too and very high quality for the most part and hostels are the best place for booking excursions cos there usual clientele have little money(as opposed to plush hotels) so they do bottom mid to top end prices.
North Island
Depends on your time...Auckland is like every big city but nice harbour( and if you are a U2 fan you have to go to One Tree Hill(not much to see just one tree on a hilly mound in a park full of sheep

)
The ferry trip between the two islands is lovely itself
Wellington is nice
Rotorua is fascinating and well worth visiting...hot pools of mud, geysers purple and yellow ponds of acid...like a different planet but it does smell like rotten eggs
Bay of islands(north of auckland) really nice
And loads more all over the two islands...you will have a great time and bring an umbrella...its a little like home that way...four seasons in one day