Two weeks? Three weeks? Just the North Island or the whole lot top-to-bottom? Don't try to cram too much in - the roads here can be slow, you'll probably be getting over jetlag, and in some places it's hard to get food after 3pm!
100% Pure New Zealand is great for ideas, though their offering of 59 itineraries may be confusing. Here's my more limited set.
The classic 'down the North island' trip:
(escape from) Auckland
Hamilton (Gardens)
Matamata (Hobbiton)
Rotorua (Geothermal and Māori Culture)
Taupō (Huka Falls and Lake)
Desert Road to Taihape (Volcanoes)
Bulls & Sanson
Wellington