Disclaimer - Fiji is always top of my list. I 100% recommend Likuliku resort in the Mamanuca Islands.
Rarotonga (Cook Islands) is great for a holiday where you "can do stuff", and also relax.
We stayed at the Rarotongan Beach Resort when I last went.
Bikes, village, snorkel, fish, scooter around the island, resort hopping etc. there is plenty to keep you occupied, if that's what you want.
It's a good sized island, and there is lots to see and do.
Go to Trader Jacks for a drink or three.
Watch out for the fa'afafine.
Getting a drivers licence is funny. You need a scooter to do the test, but they won't rent you one unless you have a licence (go figure!). Get the resort you are going to stay at to organise one for you.
They use NZ notes and their own coins.
The mountain in the middle of the island is incredible.
It's bloody hot, and you will burn.
Definitely go to Aitutaki for at least one night. It is well worth the cost and effort. (It's a short flight on a tiny plane).
Muri Lagoon - organise a tour or fishing.
There are two buses - clockwise and anti-clockwise. They get really full, and if they are too full they will not stop.
The 10 BEST Restaurants in Rarotonga 2019 - TripAdvisor
THE 10 BEST Things to Do in Rarotonga 2019 - Must See Attractions in Rarotonga, Cook Islands | TripAdvisor