If you are planning on renting a car, I would always do it on a weekly basis from Airport Rental Car companies, which usually are branded as Hertz, Alamo, National, Avis, Budget, Thrifty, etc. You can always get ultra cheap prices and do one of two: 1. negotiate the rental car insurance price to cover for everything 2. use the European point of sale (put France or UK when on website) and make them include them in the price at huge disscount.
Now, always remember to park at correct area, as cops like to take away license plates instead of just putting a ticket on the car (tickets are not paid by drivers as there is no system that can make you). if you always park on designated areas at streets in PDC, you will always be OK. Sometimes that involves walking two or three blocks to Hotel and parking for free all night. If not, there is tons of rental car companies through 5th Avenue branded and not branded.
Also, on the SIM deal, buy a prepaid SIM card from Telefonica or Iusacell with access to 3G data for no more than $300 Pesos or $25 USD with 1GB, enough for a full week of web surfing from smartphone. I always take my SIM card cutter in order to get a cheap norma SIM card and not get charged the extra for a Nano SIM card for my Iphone 5 unlocked from Apple Store. That I also do in Europe and Australia.
I go to CUN and PDC once a month for the whole weekend, and of course, sometimes I try to discover new things. Recently, a foreign girl took me Snorkeling to Akumal, which is between Tulum and Playa del Carmen. Tons of sea turtles and beautiful reefs. They usually rent snorkels for a small price, but if you want to save money, buy your own snorkel from Walmart PDC and take the Mini Vans that charge you $4 USD that depart 2 blocks behind the ADO station. You can spend half a day at Akumal snorkeling and sun bathing at that amazing beach and spend close to nothing.
On the other hand, if you want to spend more money, then Xcaret and Xel-Ha will do the trick, as others have recommended.
Enjoy your ultra cheap massage at the beach. Usually $20 - $30 USD for two full hours, depending where you go to from Canibal Royal-Playa Mamitas side to Ferry Departure Beach side.