Half an hour by ferry out into the Hauraki Gulf, Waiheke Island is a fantastic day trip for a change of pace. Sea Kayaking, cycling, tramping or hanging out on the beaches, if the weather is good (and it's usually better than the City) you can't go wrong.
Fullers ferries operate services throughout the day departing from the Harbourfront.
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Worth staying a couple of days; you can easily rent a house. There are also a couple of good vineyards with restaurants. I agree; Cable Bay vineyard is especially beautiful with a view over Auckland from the restaurant's manicured lawn. Maitre d' Ilya is a good laugh.
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