Playa del Carmen is a good choice, I think. It's well situated so you can do a day or two of diving in Cozumel if you like and have easy access to cenote diving and other mainland activities. The walls and swim-throughs on Cozumel are impressive and you'll see some large marine animals since it's a protected national park. The lobsters and groupers can grow fairly large. However, there is more variety in marine life in Playa and most people are surprised at how much they see on the Playa reefs. You can see turtles (hawksbill, green, loggerhead and with luck a kemp-ridley) all year long, nurse sharks (saw a 6 ft one today), lots of fish, seahorses/pipefish/pipehorses, squid, octopus, and lots of other stuff from the large to the very small. If you go to Playa between November and February or so, can often see a bullshark or two on your dive.
The cenotes are not for everyone (issues with claustrophobia with an overhead environment, lack of tropical fish are some reasons divers don't like them), but if they are something you're interested in, do dive them at least once. A lot of divers plan on just one cenote dive trip, but are so amazed by them that they change their dive plans. The visibility and light show are incredible - an unworldly experience!
As for the ruins, everyone has different styles. Chichen Itza is large, and can often be very crowded, but is pretty cool to see. Ek Balam is a nice alternative. If you don't want to pay for a flight to these places, there are other tour operators or private guides who can take you. Of course, that takes more time. Another place to consider is Coba, 45 min outside of Tulum. It's low key, in the middle of the jungle and you can hike or bike it. It also has the only pyramid that you can still climb. I like Coba for the greenery and the lack of crowds.
