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. Remember though...there's no evening service on Sundays.