Haven't read all the other posts but I will fill you in on all the things I have done there. Took two trips, so this wasn't all crammed into just 3-4 days haha.
Horseback ride.
You are paired with a horse by riding experience. They say you are going through the "jungle" so do NOT expect a beautiful rainforest. Did see lots of papaya, which was cool. Also saw a freshwater crocodile hanging out in a small cenote. This was about 20-30 minutes. Then you ride down the beach. No galloping, just walking. After a stretch you stop, dismount, and have the option to ride a horse in the ocean. They took 2 horses and the worker/guide drags the horse with a rope through the water with you on its back. They also take a picture you can purchase at this time. Finish on Maroma Beach which was a great place! Tons to do there, GREAT authentic Mexican food. Our ride picked us up about about an hour and a half to relax at this beach. ATV tours can end here as well.
Altournative.
EXTREME TULUM
This was the major ruin that was closest to our resort, so we chose to do this one. You take the jungle hike, see Mayan villages, ride ONE zip line through trees, repel into a cave, snorkel in that cave's cenote, and there is a zip line from above ground down into the water of the cenote. We got to ride that about 3 times, very cool, but huge wedgie. Then lunch cooked by Mayans (it was delicious chicken with sides). After that, went to Tulum. It was gorgeous, but everything is roped off. You can explore, listen to the tour guide, and/or go down to the beach (which was crowded). Pretty much an all day thing.
EXTREME COBA
Same type thing. Drive by Mayan village, walk through jungle (very uneven ground, be careful, my aunt fell), ONE zip line across a large pond that is full of crocodiles (pretty cool), lunch, repel into cenote (you literally are sat in an intertube, float in one place for awhile, then they pull you pack up or you can climb a rope ladder... I chose the ladder). As far as cenotes go, Tulum adventure was better. Then lunch (I barely ate, the food wasn't as good in my opinion), but we went to Coba. It is not on the ocean as Tulum is, BUT it is the LAST Mayan ruins in the area you can actually climb. Chose it over Chichen Itza because of this (and it's not as far). It is literally a whole city and you get to climb to the top of what is the tallest Mayan structure in the Yucatan (only tallest is in Guatemala). So that was the upside to this. You can rent a bike for $1 to get around this place.
Parasailing.
If you've done this before, no need to do it again. Exciting going up, just hang out in the air and it's kind of like okay it's pretty but what do I do now? The guys started dipping us in the water and shooting us back into the air, which was fun. The wind was wild that day so we were smacked by large waves. This was the people out of the marina at Puerto Adventuras.
Shopping in Playa del Carmen 5th Ave.
If you are souvenior bargain shopping, go. Lots of stuff gets repetitive but there are tons of bars and restaurants as well.
XelHa.
Very cool place. Mango Margartias were great. Snorkeling was so-so, I felt it was better at our resort (Catalonia). The tubing was fun, you start off going through some smelly mangroves that you aren't supposed to touch ["Do Not Touch the Tangle"] but that was impossible, haha! There are stops you can make along the way such as cliffs to jump off of, and one had a rock climbing wall that you can try to get back up with. Single and double tubes. Also have Snuba & Dolphin Swim there. Tons to do. Hammock island was cool, but who has time to just sit there? Good spot for hungover people. You have to buy their Eco-sunscreen. Bought some in Playa but they said it wasn't the right kind. Oh well.
Zip Lines.
We didn't get to do this because the place wasn't opened yet, BUT there was a zip line park being built to open December 2009. It has peaks and perches all over and if you want to ride multiple zip lines, this seems to be the place to go. Definitely want to check that out when I go back.
This got lengthy, but I wanted to be sure to give enough detail for you to find what you want. If you have any questions, let me know!