I visited Rarotonga February 2019 for 16 days.
Fantastic place. An absolute paradise island. We spent 50% of our time snorkeling / on the beach, and the other walking, exploring, etc. There's loads of amazing hikes, snorkeling is breathe taking and water is like a warm bath (really, the warmest sea water I've ever experienced).
You can do a lap of the island in 30 minutes (we got a cook island driving licence and hired a moped) or hop on the bus to go anywhere on the island (clockwise or anticlockwise services) so I would suggest staying south / west, in the quieter parts of the island and commute to main shops etc.
Link below to the home we rented. C. £100/night. Hosts were amazing bringing fresh fruit, bread etc.
https://www.booking.com/hotel/ck/honey-rose-retreat-rarotonga.en-gb.html
Price wise, it's fairly expensive probably 1.5x price in the UK for general goods, food etc. But not too bad.
All comes down to budget and length of stay but you can't go wrong. Do it.