Johnny Cay has perhaps the most perfect beaches you will ever see. The sand is soft and white Coral sand that doesn't heat up in the sun. There are palm treas just off the beach for shade and Coconut drinks and grilled food available from shacks on the Island. Just out to sea the beach turns into interesting rock and coral formations that hide very cool fish definitely a fun place to be.
Rose Cay AKA El Acuario
The "Acuario" Cay is a little Island off San Andres that attracts a huge variety of fish. This is the easiest snorkeling and fish you will ever have to do, as the fish are quite used to the people they get scarily close. Even if you don't snorkel, actually, even if you don't know how to swim, you can wade out into the water, use swimming goggles and see an amazing variety of fish.
West View AKA Natural Pool
The area known as the Natural pool has water so clear that you can look down from the cliffs above and clearly see fish swimming under 10 FT of water. If you go bring your bathing suit as you can jump in from about 10 or 15 feet up and see a ton of fish attracted by bread crumbs conveniently made available on the path to the cliffs for a small fee.
If you are not into the hordes of tourist, this beach might not be for you on a weekend, it has the nice long cresent shape and soft white coral sand, dead center in front of the Beach is Johnny Cay, looking like such a tropical paradise that you almost forget that you already are on a tropical paradise.
Half the beach receives shade from the palm trees, so renting a tend is not really necessary unless you have delicate skin; occasionally a woman will come by balancing a plate of delicious treats on her head, make sure you don't miss the delicious Coconut treats called "cocadas".