The highlights of my visit to Rangitoto were the views and the lava caves. The scoria terrain at the base of the island was very cool too. The crater itself is full of greenery, and the volcano is inactive, so don't expect a rocky, steaming caldera.
At the base of the island, near the jetty for the ferry, there are a bunch of "historic" baches. Some are still inhabited. It is hard to see what is historic about them, though they do add a touch of local flavour.
One more thing to note is that the dark, volcanic rock absorbs and radiates a lot of heat from the sunlight. At the rocky bottom of the volcano where there may not be much of a breeze, it can get quite hot.
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