Turtles. How many places can you just decide to snorkel off the beach out front (in our case since we were staying right there) and see turtles? Not once, not twice but three times. This was my husband doing the snorkeling (I do on occasion but it's really his thing - I enjoy time out with a fabulous view and a good book - also provided... fab view that is). The turtles came as a combo deal with a huge variety of other marine life - he was thrilled. Hopefully I'll be able to attach at least one of only two pics he managed to take of them before our, previously trusty, underwater camera decided to have a fainting fit and die for the rest of (meaning most of) our latest visit to lovely Rarotonga. Really annoying but it doesn't take away from how wonderful this place is for the dedicated snorkeler - he just had to go and enjoy the fantastical underwater world sans camera after that.
Absolutely recommended even if you have to bus there to enjoy. There's public access opposite the well known Raro land mark of Fruits of Rarotonga if you aren't lucky enough to be staying beachfront in the locale as we were.