Hard to say until you narrow it down a bit. Only 2 weeks does not give you enough time to see much but, for a start, there's...
Manapouri Power Station tour. We have done this and it is really interesting, even if it doesn't sound like it :sml2: . Not for the claustrophobic as you go 2km underground.
https://www.realjourneys.co.nz/en/exper ... r-station/
Fiordland National Park Day Walks
http://www.doc.govt.nz/documents/parks- ... -walks.pdf
Milford Sound Discovery Centre. Has a floating Underwater Observatory. Brilliant! See rare black coral and all sorts of stuff.
http://www.southerndiscoveries.co.nz/mi ... bservatory
Albatross Colony – Dunedin. Also has a gun battery. Very interesting. Don't shoot the lighthouse!
http://www.albatross.org.nz/
Visit Stewart Island – lots of walks, including the Rakiura Track. Fly or ferry over. Not cheap but worth a look.
http://www.stewartisland.co.nz/
http://www.newzealand.com/int/feature/rakiura-track/
The Catlins offer walks, cycling trails, fishing, kayaking, penguins, horse treks and lots more.
http://www.catlins.org.nz/
Walk a bit of the Routeburn Track
http://www.newzealand.com/int/feature/routeburn-track/
Walk a bit of the Milford Track
http://www.newzealand.com/int/feature/milford-track/
Walk a bit of the Kepler Track
http://www.newzealand.com/int/feature/kepler-track/
Lots to do around Te Anau...
http://www.lonelyplanet.com/new-zealand ... -anau-glow