A bike ride around the island only takes a few hours - Rarotonga just isn't that big. Just follow the main road along - you'd have to work very hard to get lost. And although it is the main road, it's not too busy.
It's a good way to see more of the island and you can take your time, stop off at a beach and a little cafe for lunch. You'll get the chance to see what the local houses are like (and count the number of graves in the backgardens - Rarotongans often bury their relatives at home).
One thing I'd really reccomend, make sure you get a comfortable bike. A lot of the rentals are old bone rattlers and after a few hours in the saddle you'll really feel it.