Hi,
Really depends on what do you wish to see/achieve/do/experience?
I was just back from NZ south island (and a wee bit of wwoofing in north island) and here's my itinerary of south island:
*Oh, worth noting that while I was traveling around, there are an equal amt of people who do the anti- clockwise direction vs. clockwise direction. Here's similar to what I did (except that I skipped Mt Cook totally):
- Wellington -> Picton [main gateway into South Island via Marlborough Sounds, have to try]
- Picton -> Nelson (nelson's near to some boutique family run wineries such as Neudorf and Seifried)
- Nelson -> Blenheim (great base for exploring Marlborough wineries, worth visiting: Hunters, Hans Herzog, Seresin - a biodynamic vineyard and of course everyone knows Cloudy Bay)
- Blenheim -> Kaikoura (you have to.. have to.. try the crayfish)
- Kaikoura -> Christchurch (seal colony viewpoints along the way)
- Christchurch -> Lake Tekapo (powder blue lake from the residual of ice age times)
- Lake Tekapo - Mount Cook
- Mount Cook - Oamaru (for another chance at penguin colonies along the coastline) -> Dunedin (for its Albatross and penguin colony in the Otago Harbour area)
- Dunedin - Invercargil (try the bluff oysters. on the way, have to do the drive into Catlins Forest Park, Kaka Point, head to Curio Bay where you have a chance of seeing hoiho-yellow eyed penguins in their most natural habitat.. w/o paying.

)
- Invercargil - Te Anau (for the section of Milford track, or most people do parts of the Kepler Track)
- Te Anau - milford sounds
- Milford Sounds - Queenstown (bungee, shotover river, para-gliding.. not called adventure city for nothing, more wine!!!, Fergburger, a panoramic view up the QT Hill which warrants a 1hour uphill track)
- Queenstown -> Arrowtown (beautiful quaint little town) -> Wanaka (visit Rippon Vineyard)
- Wanaka -> Fox Glacier (where you can do the ice walk treks to FJ)
- Fox Glacier -> Greymouth
- Greymouth -> Christchurch (if you have time, visit Akaroa peninsula, said to be a french little peninsula, i did not have time for that as it was time for me to return home!)
*One thing to note, post last year's earthquake in Christchurch, pretty much most of the stuff/action within city centre (now known as red zone) is out of bounds. Having said that, the area outside red zone is worth visiting just to have a tour of the quake-ruined houses and the quirky makeshift containers which are used as malls known as the Re:Start.
Writing this out of what I can recall, if you need further information, i'll be happy to share my experience.
FYI - I was backpacking alone and bought a cheap car in Blenheim for 1.2K NZD. served me well and beats renting a car, though you have to live with the risk of a 20+ year old car breaking down. Hitch-hiking and picking up hitch-hikers along the way is generally safe. ( they say if you want to start hitchhiking, NZ is the place! By statistics, i was told that less than 5 backpackers disappeared in the last 10 years. do your math) Promote the hitch-hiking spirit and pick up people along the way, always refreshing to hear stories and travel tips which may be useful, or if not, an added experience. I have met heaps of people along the way and have definitely enjoyed hearing different stories each time.