David, it's hard to go wrong being anywhere in the south Island!
I've only been a few times and it's always jaw dropping. One of the most incredible things I've seen in my life was the pebble beach at Kiakora. Billions of pebbles, gorgeous water, and a giant mountain going right into the sea. I'd definitely check that out. AlsoMilford Sounds. There are a few kiwis in here that should be able to give you better advice. If you make it up to Auckland look me up.