We just got back from a week in a Villa about 15 minutes North of Tulum. We found it to be really beautiful (but hot, imagine - Mexico is hot in August!). Wear a swimsuit to Tulum, there is a beach right below the ruins to play in. Tulum is basically off the south point of Cozumel - probably about 40 minutes south of the Playa del Carmen ferry. There is a temple and a few additional buildings to see. Admission is cheap, but get a guide (which will be more expensive). It took us about 40 minutes to see Tulum with a guide, and we spent another 40 exploring by ourselves.
If you are doing this as a "on your own" excursion, there is a great snorkle lagoon out of Akumal called Yal-Ku. Easy snorkling because of the lagoon (though the roads will take your car out!). But the snorkle lagoon closes early.