If you are energetic, I would get a taxi to Cornwall Park and walk around the park and up to the monument to see the view. Auckland rests on a number of dormant volcanoes so there are lots of these parks with great walks and views. Alternatives to the above include Mt Eden and North Head. The latter can be accessed from the Devonport Ferry. Which reminds me - instead of Waiheke, a shorter ferry excursion is to Devonport which is a quaint place also with great volcanic cone walks. All the ferries run from a terminal that is about 100 yards away from the cruise dock.