As a travel agent I get asked a lot of itinerary ideas. Here's a few possible places around North Island in NZ, enough for 2-3 weeks, depending on your preferred pace.

• Around Auckland - Rangitoto Island and Waiheke Island, both ferry trips from Auckland CBD. Rangitoto is a volcanic island and Waiheke is known for its vineyards and beaches. 

• North of North Island - Bay of Islands and Waitangi (where NZ was founded), Cape Reinga (where the Tasman Sea and Pacific Ocean meet), Coromandel and Cathedral Cove (lovely walk, hot water beach)

• Must, must - Rotorua (hot pools, mud pools, geysers, Maori village experience (Marae, haka, hangi)), Waitomo Caves (glow warms), Taupo (largest lake in NZ, Tongariro Crossing - amazing all day or even three day trek (Mount Doom of Lord of the Rings!))

• Napier (Art Deco city, resurrected after the earthquake in the 30s, wine region), Wellington (Capital of NZ, artsy city, Te Papa museum, day trip to Picton on ferry?)

• Western Coast - Mt Taranaki (a challenging but doable tramp course, the mountain that got banished from the Taupo area by Tongariro)

I prefer the South Island more - I'm biased because I'm from the South, but this particular request was for someone who had already been to the South. 

For more tips, visit my Roadtrips page as well.

NZ is a beautiful place. Please drive slowly to enjoy the view, and enjoy the scenery!