On our recent trip to visit the MIL and one BIL, we took a side trip to the beautiful Island of Aitutaki. DH and I had thought about visiting Aitutaki as part of our honeymoon in 2007, but we have expensive tastes and decided to go at another time when we could afford it more comfortably!
We stayed at the beautiful Aitutaki Lagoon Resort and Spa in one of their over water bungalows and it did not disappoint. http://www.aitutakilagoonresort.com/
While the resort and surrounding area was beautiful (especially the sunset at the bar), the snorkeling was truly at best in the main lagoon. So on one of our days we took a boat trip with the Aituatki Adventures and their yellow boat! It was a fantastic day Island hopping, snorkeling and of course the wonderful fish BBQ lunch.
So if your visiting the beautiful Cook Islands (which I highly recommend), I suggest spending some time in Aitutaki. While I'd love to say we'll be back when we next visit the family, unfortunately there are 15 Islands all up and I think we'll be visiting Aitu or Mauke.