Miguel
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This island is awesome because it's for everyone - hikers, campers, day trippers, family, etc.
From Paihia/Russell, you can take a ferry ($50 return) to Otehei Bay.
You can spend the day then go back. Or much better to stay a night or two to camp. I recommend walking to Urupukapuka Bay which is around a 15 minute walk from Otehei Bay and camp there. The view is better and there's a lot less people. There's a toilet (and an area to shower I think)
Bring your camera. At night we saw the milky way as the sky was clear.
In the morning you can hike, there are a bunch of hiking trails on the island you can pick from not very challenging as there's not a lot of ascent.
Back at Otehei Bay, there's a restaurant - they serve food and BEER!