Hello Gilles & Carolle,
Welcome in advance to NZ.
Wings over Waiarapa (
http://www.wings.org.nz/) would be a must see if you can work it in. Not too far from Wellington. Peter Jackson's museum at Blenheim also. A nice ferry trip to this one.
If you get as far afield as the Bay of Plenty we can show you around. Just let me know.
Regards, Terry