You might consider Playa Corona. It has a sandy area with palapas, though there are a couple of steps down to the water. The snorkeling is pretty good, with lots of sea fans and stuff in shallow water. There is an an outcropping of staghorn coral 100' or so north of the building, roped off but you can get pretty close to it; it's the only staghorn I know of that is accessible from shore on the island. There is a restaurant and bar there, although I have never gotten anything there besides a few cervesas so I cannot attest to the food selection or quality.
There's no day charge but they of course prefer that you buy something. It's a bit off the beaten path; every time I have been there there were only a few other people there. There are no bouncy float toys, parasailing boats, jetskis, pod people, loud gringo music, etc., all of which are pluses for me. It's just a funky little beach club with a friendly staff.
It's a little south of Chankanaab.