Got some info on the South Island now. We travelled much of what it has to offer but there are a couple of places we didn't manage, I'll still refer to them though.
South Island
Nelson and Picton region (Marlborough Sounds) - This is an area we didn't get to but have always heard great things. It is the Northern region of the Island. Again, something we didn't manage was the ferry from Wellington across the water. Apparently this is an amazing experience in itself. My missus did it in her first visit to NZ and raves about it.
http://www.visitpicton.co.nz/
http://www.nelsonnz.com/
http://www.marlboroughsounds.co.nz/
Kaikoura - THE place to go in New Zealand for whale watching.
http://www.kaikoura.co.nz/
http://www.everythingnewzealand.com/kaikoura/
Christchurch - Beautiful city, sadly much was destroyed in the earthquake. You are allowed to travel into the 'red zone' now, which is a bit eerie. They have a lovely botanical gardens and the city reminds me in some areas of York. There is a pretty cool area in the city centre which is a bit like a flat-pack retail and entertainment area, built after the earthquake so business could keep running.
http://www.christchurch.org.nz/
Arthurs Pass - Another area we didn't manage to get to, but as ever have heard amazing things about. Very LOTR.
http://www.doc.govt.nz/parks-and-recreation/national-parks/arthurs-pass/
I believe you can take a Tranz-Alpine Express from Christchurch across the width of the country, stopping at Arthurs Pass
http://www.kiwirailscenic.co.nz/tranzalpine/?gclid=CIeGh5ro57YCFQhjpQodzGEAog
Lake Tekapo and Pukaki - We did both these lakes in the same day as we were travelling towards Queenstown. Stunning emerald lakes and at Pukaki (I think), you can travel up to a mountain top cafe with exquisite views across the region.
http://www.tekapotourism.co.nz/
http://en.wikipedia.org/wiki/Lake_Pukaki
Franz Josef Glacier and Mount Cook - One of NZ's big big hitters. Absolutely awesome.
http://www.franzjosefglacier.com/?gclid=CPydkbbp57YCFcgbpQodLD0A0g
http://en.wikipedia.org/wiki/Aoraki_/_Mount_Cook
Queenstown - Stunning views, great night life and possibly the number one destination on the planet for adventure activities! Whatever you want to do, can be done here. There is a cable car that goes to a peak where you can chill out and take in the views, go on walks and also race down a karting track. A must-do if you are in the South Island. Lot's of travellers come here, including many lovely European ladies of questionable virtue
http://www.queenstownnz.co.nz/
Milford Sound - Possibly my favourite destination in NZ. A huge Sound, with cliff peaks three times the height of the Empire State Building. Also one of the natural wonders of the world. It's a bit of a drive to get there, as you have take a U-shaped journey from Queenstown, if that's where you're going from. Well worth it. There are various ferry journeys you can take to explore the whole Sound, worth doing. We shared our ferry and hung out with James Nesbitt and a couple of other actors filming The Hobbit! There are also other smaller sounds that can be explored on the west coast, such as Doubtful Sound.
http://milford-sound.co.nz/
Bluff - The Southern-most tip of the Island. If you drive through it you can walk up to the tip. Probably not worth staying there, but if you're driving through worth stopping.
http://www.bluff.co.nz/bluff.html
Stewart Island - Due to time and money constraints (ferry to the island is pretty expensive each way), we didn't get to the island. But meant to be another great natural attraction.
http://www.stewartisland.co.nz/
It's a great place for Great White cage diving if you're up for it!
http://www.adventurecrazy.co.nz/SHARK+DIVING/SHARK+DIVE.html
The Catlins - If you're driving around the area, this is a huge DOC region with plenty of forest, wildlife and waterfalls. You'll be pretty much stopping every 20 minutes.
http://www.catlins.org.nz/
Dunedin and Otago Peninsula - Great little University city. Plenty to do for such a small place in terms of eating out and entertainment. Then you have the Otago Peninsula, where you can hire bikes and travel around the coastal areas. Need to be pretty fit and it is very hilly. You can also view sea lions, seals, penguins and albatross in their natural habitat. Word of advice, don't get too close to the seals and sea lions, they've been known to chase after you and are surprisingly quick! The area is also home to the steepest street in the world, Baldwin Street.
http://www.dunedinnz.com/au/home.aspx
http://www.otago-peninsula.co.nz/
Oamaru - If you're driving up the coast, worth stopping off here to see blue penguins, more seals and sea lions and a very Victorian lifestyle.
http://www.visitoamaru.co.nz/home.aspx
Timaru - Another place to stop-off at if you are driving up the coast. Great beaches and scenery.
http://www.tourism.net.nz/region/canterbury/canterbury---timaru
If you're driving, there are plenty of places to stop off and just admire the views, including lakes, waterfalls and wildlife. We spent the night in an area called Peel Forest, where we went horse trekking for a few hours. People are fantastic and as ever, very helpful.
Everything is sorted out on a trust basis. We treated ourselves to a lovely guest hose in the Catlins one night. Walked into the house, no-one was in. There was just a note saying to ring a number and sort out your stay. Then we just left the money on the counter the next day. This is a very common occurence in NZ and particularly the South Island. You'll see a lot of honesty boxes around if people are selling produce on the side of the road.
Again, only just scratched the surface of what can be done here, but hopefully it gives you a bit of a guide. One thing you must make sure you have is insect repellent. There are areas, especially the Sounds where black flies are abundant and will drive you crazy! Also, lot's of mozzies around depending on the time of year.
Enjoy!