The Hermit and Ninigo Islands.
Lots of lagoons, atolls and reefs. We snorkelled and lots of people went scuba diving (I don't know how to scuba dive).
In case you're wondering how I got all the overhead shots, the True North carries its own helicopter on board, and passengers can choose from a series of flights, available almost every day of the trip. I HIGHLY recommend this option if you get the chance.
We saw Pilot Whales too. Awesome stuff.