Always hard to do recommendations for friends arriving as not sure of their budget etc.  If they have the budget would be great to go on an organised get away to a place that is otherwise hard to experience, a e.g. Fiordland multi-day boat trip or a staying at lodge on a S.I. station.  A 4x4 trip also a great idea, this company is very good and can hire a 4x4 for the trip too: 4X4 Tours | 4X4 Explorer Adventures

The far North (North of Kerikeri) just don't leave the car unattended.

The Catlins.

Hot Water Beach in Coromandel is one thing that is always called a favourite by our visiting friends, although timing it for low tide is a must.  And those that did it say hiring the charter boat Epic at Whitianga for fishing is great.  Coromandel is a safe bet as it offers loads of walks for those that like the active side, or plenty of blob out beaches for those that don't.

There is a glass blowing gallery in Whangarei where you can watch them making their wares which is well worth stopping for, free to watch and is really interesting as they often give a commentary as they go.  If they head up north they'll be passing by.

Definitely recommend Nelson/Abel Tasman - area is great, Murchison is cool for a day if you want to do some helicopter'ing and gold panning.