Not all land transport is quite like I encountered on Aitutaki (see picture above)! The stunning scenery makes walking a pleasure, but avoid doing it when the sun is at its hottest.
On Rarotonga, the best travel tip is to take the Cook's Island Bus which circles the island, clockwise and anti-clockwise throughout the day and into the late evening. If you're going to use it regularly, you can buy a day pass or a book of 10 tickets. But be warned...there's no evening service on Sundays. Oh, and one final tip: make sure you read the small print on your bus ticket - its says "please smile"!
Motorbikes can be rented on some of the islands, provided you've bought your Cook Islands driving license from the police headquarters in Rarotonga (international licenses are not valid). And car rental firms abound on the capital island.
Petrol and diesel fuel are quite expensive though, with the outer islands more pricey than Rarotonga.
It's a bit hit and miss what transport you'll find on the outer islands. But that's the beauty of them. The local tourist information offices will help you plan your trip and offer advice on getting round when you arrive.