Just an hours drive from Nelson lies the smallest, however also the most popular national park in New Zealand, the Abel Tasman Park.
Stretching from Marahau in the south to the Wainui Inlet in the north are beautiful easy to manage walkways along the coastal track. Walk in from Marahau or take one of the numerous water taxis from Marahau or Kaiteriteri to one of the beaches, walk for a few hours and take a water taxi out again.
Alternatively stay in one of the huts or at a campground over night and continue your trip the next morning.
The whole coastal track is around 55 km long and can easily be walked in two or three days.
Far more challenging is the inland track from Marahau to the Wainui car park. On its 38 km it brings you up to more than 1,000 m above sea level, with some really steep sections. However, the inland track access from some car parks,so that it can be done in smaller sections.