I've been to NZ twice: once for a few days as part of my honeymoon, and the second time for our 10 year anniversary. The first time we just went to Queenstown and I was pretty happy with that. It's beautiful and there's a lot to do right in the area or by day trips.
The second time we spent a few days in Auckland, a couple days in Queenstown, and then hiking the Milford Track. Auckland was nice but not terribly exciting (and it reminded me of Seattle, which is where I live). We did a day trip to Waiheke Island while we were there and that was lovely. If you're just looking to relax in a beautiful setting and drink wine I think it would be great for that. Queenstown had changed a lot in 10 years, but it's still beautiful and fun. The hike was amazing. We did the luxury version which was awesome. I think we're going to go back for our 15th and do the Routeburn Track.
Travel is fatal to prejudice, bigotry, and narrow-mindedness, and many of our people need it sorely on these accounts. Broad, wholesome, charitable views of men and things cannot be acquired by vegetating in one little corner of the earth all one's lifetime. Mark Twain
I've done the south island for 2 weeks, and even that was not enough time to see it all. I think the south island is more scenic than the north (but again, haven't been there) so I recommend you pick one island and stick to it.
We did a lot. Hiked on Fox glacier (can walk on, as where Franz Josef you have to helicopter onto it so it's way more $), swam with wild dolphins in Kaikoura, did a day trip to both Doubtful and Milford sounds, went on the Shotover jet outside of Queenstown, biked around wineries in the Marlborough region. Only hiked a few hours at a time because we're not hugely outdoorsy.
We didn't do Abel Tasman national park, which I regret. Didn't go onto/up close and personal with Mount Cook, which I do not regret because there's few roads to get up it and they were a pain to reach with our itinerary.
We didn't go the camper van route but stayed in private rooms at hostels. You could go either way, but the pokey campers slowing down traffic drove us nuts on every road.
It's very beautiful! I'd love to go back and do the same thing on the north island but need to find a few grand to do it with...