|Subject: Re: Playacar|
By all means go to Cobá. And be sure and rent the bikes. There is certainly something unique about riding a bike through the jungles while transiting from one section of an ancient city to the next. Close by Cobá is the Punta Laguna Spider Monkey Preseve. Señor Canul or one of his sons will escort you on a short hike through the jungle. The monkeys are very curious and will eventually appear in the treetops above to gawk at the land-based apes. See if you can find Mexico: A Hiker's Guide to Mexico's Natural History in your local library (or at the closest university library). You can photocopy the sections on Cobá, Punta Laguna, and Playa del Carmen.
Can't tell you anything about Xcaret but Tulum was still open to wandering the grounds (after paying admission) when we were there two years ago. Entering any structures was off-limits (absolutely necessary with the hordes that pass through there now). Granted, it is more restrained than when I was there in the mid-70's but still worth a look-see.
You might consider a trip up to Valladolíd for several reasons; it's a wonderful colonial city unlike anything found on the coast, the cenote Dzit-nup (not Zaci) is well worth exploring and swimming in (the one a short cab ride out of town), and the ruins at Ek-Balam have the most remarkable stucco reliefs. I thought they were reconstructions but the ranger assured me they were the original.
About Punta Laguna and the monkeys: http://www.sas.upenn.edu/~ramosfer/spmkpl.html
About Ek Balam: http://studentweb.tulane.edu/~dhixson/ekbalam/ekbalam.html
Que vaya bién, John