We have visited Orient Bay on our own. We actually rented a car at the port for a day and kind of did a "self-guided" tour of the whole island. It was quite fun!
Orient Bay was very nice. Not too crowded, easy parking, beautiful beach, many bars around in case you wanted a drink, and, all in all, a great experience.
As far as cabs, I don't think you would have any difficulty in finding a cab to take you anywhere you wanted to go on the island.