A friend borrowed some of our gear to use on a guided walk on the Routeburn and retured it on the weekend. They had a great time.
If you want to do a guided walk, the Milford gets booked out very early. Perhaps condsider the Routeburn as a good back up.
Alternatively, Air NZ is doing some smazing deals at the moment with cheap campervan hire thrown in. Consider doing a touring holiday with short walks at various places:
Eg Fly into Christchurch, visit Port Hills and cafes of Lyttleton, if visiting ont eh weekend, check out the 'arts district' very nice.
Mt Cook - short walks, Mueller hut, glaciers etc
Queenstown - lots of things to see along the way
Fiordland - base yourself at Te Anau for a 2-3 days and sample the delights of tramping in world heritage areas....
Wanaka - is this heaven? Lots of options here.
West coast glaciers (Fox/Franz Joseph) - take a short walk on a glacier or pay $$$ for helo ride
Arthur's pass - cosy alpine village with more short walks than you can throw a kiwi at. Easy access back to Christchurh.
This would be a nice 10-14 day trip and a good insight into the South Island.
The top of the South Island also has lots and lots to see - Marlborough region wines, the sounds, Able tasman national park, Kharangi (not the right spelling) national park, the buller river....more wineries...
I'm already planning my next trip (2 little geckos on the next one!!!)
Cheers
Gerard