You might want to do something like XPLOR. That is actually down near Xcaret. It has Zip lines, swimming in a cenote, Floating in an underground cave, etc.
Xplor - Beyond the Surface -
The Ferry to Cozumel is at Playa del Carmen. Take the Ferry over to Cozumel and rent a jeep or a scooter and go around the island. Or just shop and look around. Nice day trip. Costs about $20 pp RT for the ferry.
Go to Playa and spend some time at the beach bars and do some shopping. Playa Maya on the beach around Calle 8 has great food during the day. Bad Boys on the beach has the Nasty Bastards band playing from 5-9 on Thursdays and Saturdays. Good bar. Calle 4 and Fifth Avenue is a great place called Carboncito's. Great Mexican food. Happy hour 3-6 pm with 25 peso Margaritas and 2X1 Tacos al Pastor. Exploring Fifth Avenue and surrounding area is a day in itself.
I have been to Tulum. Beautiful. But once is enough. Chichen Itza is about a 3 hour drive away. Coba is supposed to be pretty good. But both take all day. And if history and ancient artifacts aren't your thing, I wouldn't recommend it.
Here is a great website with lots of info.
Playa del Carmen info
I have been to Playa many times and will be making a couple of trips to Playa this winter. Once in January. Once in February. I do not stay all-inclusive. Beer is cheap. Food is cheap and great. Playa is a great place.
By the way, ignore the talk about Playa not being safe. It is very safe. Just don't be completely stupid. That will get you in trouble anywhere.
Just out of curiousity, what resort are you staying at?