Suggestions

Three weeks to see the South Island!!! I have lived in the South Island for about 30 years of my life and I am still trying to see it!! :-) There are hundreds of 1-2 day trips that you can do. It depends on whether you want to stay in huts or are prepared to camp, or how rough a track you are prepared for. One of the most fantastic places I have been to is Moraine Creek off the Hollyford Valley. It will be a day up, camp out and a day back. It is an awesome place. Also while in the area, stop at the eastern portal of the Homer Tunnel on the way to Milford Sound and climb up to Homer Saddle, it is probably a half day return trip. The views from up there are just fantastic.

Geez - where to start? Really, there's a lot of stuff here :) Read about the different National Parks on the DOC website: http://www.doc.govt.nz/ They do very good regional leaflets with short walks described and you can pick up hard copies from the visitor centres. Most people, me included, are blown away by the Mt.Aspiring region and nearby parks, there's a lot of choice from Wanaka to Glenorchy to Milford/Te Anau. But everywhere has it's own special appeal and I also like Kahurangi and Nelson Lakes. Just make sure you don't spend half your holiday in the car trying to tick off all the places - it's a very very common trap :) Also take a look at a road map - your options for routes are limited as there aren't many ways through the centre of NZ so you'll be doing a circuit. But get out of the city as soon as you can - NZ does wilderness best.

Of course, it depends which way you're travelling. But here are some ideas. The West Coast and Queenstown are the best bets for great short trips. I'd suggest: Welcome Flat (2-3 days), overnight on the Routeburn (you can camp in the Greenstone Valley or the Route Burn North Branch) (2 days), Fox River (2 days), Heaphy Hut on the Heaphy Track (2 days), Ben Lomond (1 day), Mount Fyffe (1 day), Mount Somers Track (2 days), Mueller Hut (2 days).

Thanks for the suggestions, will do some research on the doc website about the tracks mentioned, cheers everyone