I used to work/live in Rarotonga for 6 months in 2014/15. I'll agree with a comment earlier in the thread, that it is an amazingly beautiful island. One of the prettiest I was fortunate to call home for a while. A very friendly population.
Diving wise, I would certainly do a dive while you are there, but I wouldn't call it a dive destination as such. The diving has amazing visibility, some marine life, and mostly hard corals. The diving on the north side is mostly shallow-ish coral reefs ending at a drop-off. The south you dive the passages in the reef. These passages offer a good chance of seeing bigger life, very regular sightings of white-tip reef sharks, eagle rays, schools of jacks and barracudas.
Jun - Sep is the time for humpback whale encounters.
Dive operators on-island
The Dive Centre, The Big Fish - Rarotonga, Cook Islands - Padi 5 Star Dive Centre
Scuba dive the Cook Islands with Dive Rarotonga PADI 5* Centre
$99 2 dive special! PADI 5 Star IDC centre. FREE online learning with OW course
The Dive Centre and Dive Rarotonga are more on the SW side of the island, whereas Pacific divers is in Muri.
I'm not personally familiar with diving in Aitutaki, I'm led to believe is more shallower diving in general.
A must for top-side activities is the cross-island walk!
Some favorites for a meal and drink: Vaiana 21.3 beach bar, Islander hotel for cheap happy hour cocktails, Muri night market, Shipwreck bar, Boogies on Friday night, then dance at Rehab, Saturday morning market in town, Castaway bar, dinner at Vaima (must), snacks at Charlie's, Maire Nui Gardens and Cafe.
Be aware that alcohol checks are very common at night, so if you plan on having a big night out use a taxi service or one of the private 'party busses' that continue the party while they bring you home after the bars close.