I know a lot of people travelling to NZ either do not have much time to see the sights, or underestimate the actual size and scope of the country.
Does anyone have suggestions for itineraries that fit in as much as possible in a certain time?

Bearing in mind that the most spectacular scenery is generally well away from the cities, I tend to suggest back roads and more rural accommodation when I'm asked where to go.

Here's one from me:
3 weeks in NZ, rental car or campervan.
[Day 1] Fly in to Auckland (generally most convenient)
Stay one night if necessary, to find your feet. Time in the morning to have a nosy round the city and check out galleries and museums - tomorrow's not a long drive.
[Day 2] Drive south to Waitomo (195km, 3hrs)- the cave formations and glow-worms are pretty cool. There's a B&B on the Waitomo/Main Road corner (Big Bird) that has an ostrich farm, and Ross there used to be a Dept. of Conservation ranger and has a wealth of knowledge about local flora and fauna. He might even take you to a nearby non-commercial glow-worm cave and bush walk!
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