Take a cruise in the Abel Tasman National Park. These take you to starting points / pick-up points along the various trails in the park. There are a number of walks you can do, ranging from 5 day-long to a few hours.
One of these cruises, we went past this unique cluster of stones in the water called Split Apple Rock. It looks like a big round stone had been dropped right there, cracking itself in two.
Tonga Bay Beach among one of the many beautiful stretches of beach in the Abel Tasman National Park. We kind of just stumbled on it on our hike through the bush.
A very tranquil and peaceful place. Admittedly even in the summer (late December) there was no way I would have gone in the water...far too cold. But many people were out canoeing and stuff.