I would suggest a day trip up into the Able Tasman National Park.
There is a brillient water taxi service and you can walk sections of the track if you want(or not) They will drop you off an pic you up at different bays if you desire to walk between. Track is really well maintained and more like a footpath/side walk guess you Americans would call it.

http://abeltasmanseashuttles.co.nz

or these are the guys we usualy go with

http://aquataxi.co.nz