With only 32km around Rarotonga and literally ONE ROAD that covers the circumfrence of the island, there certainly is a lot of inexpensive choices around Rarotonga. My personal favourite is by scooter or bicycle. With the roads being relatively flat, it really cannot get any easier. The only complication would be going to the Police Station and getting a Motorbike licence if you have not got one and setting the practical or theory test. It is not that it is difficult, but just a bit of a waste of half a day, but it is almost worth it.
Cars Cheap and Cheerful
There are a great choice of cars available on the island. Small, convertable, vans, large SUV's and the latest minis. From $55 a day, well worth investing in if you are a group of 3 or more. Pacific Cars, Island Car & Bike hire, Polynesian Rentals are all within walking distance to the property. You only need your overseas license to drive in Rarotonga.
Catch the Bus
Great thing about Cook Island buses, is that they run all the way to 11pm, only cost $5pp each way, have Clockwise and Anti Clockwise. There are bus passes you may purchase if you plan on catching the bus a few times and one ticket may be shared amongst two or more persons