In NZ, we enjoyed the South Island better than the North if you are limited on time.
Our favorite quaint town in the South Island was Queenstown. The boat tour on the Milford Sound in Te Anau was pretty. Hiking in Abel Tasman. Kaikoura dolphin watching boat trips.
On the North Island, we liked the town of Mount Maunganui - glow worm tour. Paihia quaint Oceanside town far north - dolphin watching.
My views: I was in NZ about six weeks last year, rented a car and stayed mostly in these - great value. Note that they are geared towards shared accommodation but they all have lovely people/owners running them and a great atmosphere (except in the main centers like Auckland and Wellington).
https://www.bbh.co.nz/ If you do go to Te Anau, try and stay with Bob and Maxine. They probably won't remember me but they are fantastic people. Bob really helps you with hiking planning and such.
I haven't seen even 50%, could easily spend many months there.
+1 on the South Island, however consider Wanaka instead of Queenstown. It's more laid back. If you like hiking anywhere is good, walking around Te Anau and doing a great walk or two is a no-brainer. However, these tend to be booked full (for good reason) in peak season.
Great Walks: Tracks and walks .If you have a tent, spots are easier to get. Watch out for bringing outdoor equipment with you though.
Everything is supereasy and well marked. Don't plan too much ahead with the exception of the great walks if you choose to. iSites tourist center is there to help you in every little town.
A flyover of Milford Sound is great (from Te Anau, Queenstown or Wanaka), and if you can stay overnight there. It's amazing. Beware the sandflies: think mosquitoes but by day and very aggressive. They don't come out at night. The weather is very fickle in this area, keep that in mind. Fiordland has insane rainfall.
The glaciers are underwhelming since they melted. Ok to skip. Glow worms are everywhere btw, so if you miss the advertised spots ask around. On many multi-day walks there are glow worm spots closeby, ask the ranger.
One neat thing not mentioned often: if you see a turnoff to a lighthouse, go there! There's usually alot of wildlife to be seen, penguins, walruses etc .. Especially on the east side of the south Island (Catlins and above).
East of Christchurch you have Akaroa. Fantastic for dolphins and great atmosphere in summer. There also is a penguin wildlife sanctuary, well worth it.
Just a few things. Oh, and if you hire a car: get your reservation in early enough. Prices go up insanely and NZ is in a huge tourism boom (deservedly).
