Spent Christmas there a few years ago. Amazing place!
Did all my dives with the dive center (the big fish)...
http://www.thedivecentre-rarotonga.com/
They were great to us and as a bonus, I could walk there easily from where we were staying.
We stayed at Lagoon Breeze villas which was also fantastic.
http://www.lagoonbreezevillas.com/
We self catered and it was setup perfectly for that. They provided a light breakfast each morning but other than that meals were up to us.
Be warned that groceries are pretty pricey there though!
I am so jealous of your upcoming trip, I am really hoping to get back there again. Its awesome!