considering Playa del Carmen for economic reasons
PDC is a pretty good size city with 20 miles of coastline. I'll concur with and add some of my suggestions to what others have said.
If you're into cenotes (even if you're not), a day of 2 dives in a cenote is an experience. They're not my thing - I like the ocean - but if all that was available were cenotes, I'd dive them - there are fantastic geologic features to see.
The reef along the coast is nothing special, IMO. Not much, if any (as I recall), color and not super fishy.
I would suggest, if you go, to find a resort that has a dive op on site, unless you rent a car and don't mind driving. A dive op, such as Dressel Divers, is located at several resorts and also picks up from different resorts. They offer different packages that can include a day trip to Cozumel for 2 dives, as well as cenotes - prices are reasonable. But definitely do a day trip to Cozumel.
The first time I dove in PDC, I took a taxi to Puerto Morelos (about 15 minutes north from where I was staying) to dive. I would recommend this over diving in PDC. Saw more life, reefs, and a nice wreck.