If staying on north island, the Bay of Islands around Keri Keri and across to Russell is definite must. If you have time go up to Cape Reinga and along the beach highway - probably best to do this on a day tour in a four wheel drive coach.
Chocolate factory at Keri Keri has some nice delights.
Be careful driving as the roads are nowhere as good as ours - highways are not as wide as ours and curves much sharper and not as well cambered.
It takes at least 3 hours to drive from Auckland to Keri Keri.