Kiddle Bay
Kiddle Bay is a little different from your average beach. Instead of sand it is made up of small smooth stones and bits of coral. Probably at least partially because of the beach material, and also because of it's location, it is one of the less populated beaches on the island. It is located on the south side of the island, just west of the salt pond.
This is a very rustic beach, meaning there are no facilities or amenities available. If you want anything to eat or drink, you'll need to bring it yourself. Parking shouldn't be an issue since there's rarely more than a couple people at the beach.
You can also walk out along the point which will give you great views of the ocean and also looking back towards the shore. Exploring the salt pond behind the beach offers another potential activity, especially for those interested in bird watching.
Snorkeling
There is some great snorkeling to be had here. Out along the western shore of Kiddle Bay there are a bunch of large rocks, deep depressions, arches, tunnels, and all sorts of interesting formations. Coral and fish abound, as well as other creatures like rays or even squid. It should also be noted that the waters in this bay are very calm, which makes it great for snorkeling.