It's the wrong time of year for Whale Sharks or Bull Sharks, and other than that the diving off Playa del Carmen itself is only pretty good, not fantastic. My favorite sites there are the Aquarium and the Arches.
The cenotes are close by, and are excellent, but with less than 24 dives you should take a good (and honest) look at your skills before you commit to going into them. The guided tours are excellent and reasonably safe for inexperienced divers, but there are still issues that need to be considered before you hit the water (do a search, it's been discussed many times).
Cozumel is close, but if you're going to Playa to dive, I'd consider staying on Cozumel instead.
Pretty much all the diving off Cozumel is a wall. The top (shallower) portion of the reefs are relatively horizontal. Go just a bit further out and you can drop over the wall. When we're on Coz, we tend to drop down the wall for the first part of the dive and then work our way up and we approach the NDL.