Tulum and Playa are about an hour apart by taxi-van, so I consider them to be effectively the same destination.
Day 4: Cenotes
If two beach days in Tulum weren't enough for you, how about crystalline pools in the middle of the jungle?
Cenotes are basically drowned sinkholes, and the stretch of highway between Tulum and Playa del Carmen has dozens of them, each with its own peculiarities. All of them are worth a visit, but all have ticketed entries, so be sure to research which you want to see. Personally I went to the Grand Cenote for the turtles, and to Dos Ojos to swim through the cave. For any adventurous scuba fans, you are also able go cave diving in many cenotes. The last, and worst reason to check them out is that despite it's name, Playa del Carmen's beaches are actually not that beachy (compared to Tulum).
If a day in pools has left you relaxed but still energetic, the last recommendation I can give is to visit La Quinta Avenida in Playa del Carmen, which is the main tourist drag. While not the cheapest anything you'll have seen in Mexico, it is basically beachside, has all sorts of fun touristy things, and most places are open late into the night. Ranging from Haagen Dasz ice cream shop to Argentine steaks and back, nothing here is truly Mexican, but the general atmoshere has some appeal.
The Cancun airport is the most efficient way out of the Yucatan, and it gets bus service from Tulum and Playa del Carmen. One massive warning is that the airport experiences frequent delays, and it almost cost me my connecting flight back in Mexico City. If at all possible, I suggest booking your return trip from Cancun as a package so that any delays get eaten by the airline.