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Queenstown - the adventure capital of NZ. Plenty of action and adventure to keep you busy here. Plus take the boat trip along Lake Wakatipu also check out Glenorchy, for some nice scenary.
Also Wanaka is quite nice too.. great scenary, aswell as Arrowtown would be worth a stop.
Defently Franz or Fox Glacier, you could have a guided walk or scenic flight of them.
Also from Queenstown *do not* miss the Milford Sound in Fiordland! via cruise.. and even the Doutfull sound, and even do the Milford track which seems to be quite highly ranked and will be very scenic, so it may be worth doing if you had time. Just the road to the Milford sound is very scenic, Te Anau is a nice place too,
Some other walks in the South you may want to do is within Mount Aspring National park.. (lots of mountain scenary, lakes, bush, glaciers) or the Abel Tasman national park (bushland, and coastal scenary) would be some nice ones!
You may not want to go further down south than the Milford sound.. but if you do check out Stewart Island and the Caitlins i've heard there quite nice, if you don't have time give a miss. Also you may want to go to Dunedin.. though theres not much there, have a look at some buildings, cabury factory, stepest street in the world and see some wildlife. Will only need 1 and maybe 2 days there. But not a must.
Go to Christchurch, defently go to the Antarctic centre and some city things, like the boat ride along the river.. take the tram etc.. also go to the wildlife atractions such as willowbank or orana park.. southern encounter. You may want to also head out to Akaroa which is a town just out from it, you can see some wildlife and have a cruise and go swimming with Dolphins.
You can take the tranz scenic alphine train Christchurch > Greymouth, it's one of the best train journeys in the world, so it'll be worth it.
Just above Christchurch is Kaikoura, defently worth to stop by and do some whale watching - one of the best spots in the world.
Also you may want to go to Hamner Springs aswell to relax in some hot springs but this can be done in Rotorua aswell.
At the top of the South island you can go on that Queen charlotte walk if you wanted. It's near picton if you take the scenic coastal train from Christchurch.. it'll take you to picton. Also you may want to visit little Nelson and even Golden bay for some nice beaches and while you're here check out the Abel Tasman and cruise around or walk it.
From picton cross on the ferry and see some of the Marlborough sounds, you'll end up in Wellington.
Defently go to the Musuem here (te papa) it's really good! and it's not boring like most musuems! and experience some NZ city life.
On your way up the North you'll pass Mount Ruapahu for some scenary, then end up in Taupo, check out the huka falls here and maybe some other stuff. Then up to Rotorua, have a maori culture experience, and the geothermals, there not to miss. Also go zorbing and shweebing at agroventures and even the other action activites there! and also go to rainbow springs.
Then go to Bay of plenty do some stuff there.. check out some beaches.. not a must.
Then to Waitomo, defently have a guided tour of the waitomo caves!
Then up to Auckland, head out to some nerby islands, rangitoto island.. bay of islands, great barrier island, goat island, waiheke, and more, take your pick.! go up the sky tower, have a ride on a america's cup yatcht.. go to the rainbows end theme park. etc, plenty to do here.
You may want to go up to Northland, theres some stuff here and there that's worth seeing/doing up there, such as the huge sand dunes!
You may want to do some skiing and snowboarding if you're going from june-october-ish. Up in the Southern alps or Mount Ruapahu, if it's not season you can go to the indoor snow park.. snow planet in Auckland.
Some other sights/places that came to mind: Lake matheson (REALLY good mirror lake), Coromandel, Pupu Springs, Tekapo, mount cook national park, Lindis Pass, Tongariro National Park, Westland National Park