With limited time you are right to concentrate on the North Island. Bay of Islands is good as is the Coromandel Peninsula. Waitomo, Tongariro Nat Park and Taranaki all worth seeing. Id keep any visit to Taupo or Rotorua to a minimum given the time you have.
People are great. Hope you enjoy it.
Make sure you go back and see the South Island as well.

Cheers all