We stayed at Dos Ceibas for almost a week and absolutely loved it! It's an 'eco lodge' with cabanas set in beautiful gardens, right on the spectacular beach. The staff were all wonderful and very helpful. They also do yoga every morning that you can join if you feel like it. The restaurant does great food and excellent margaritas (and they will also serve you on the beach). It's a short drive away from some pretty amazing restaurants too - don't miss Simple and Posada Margarita!
The cabanas was fairly basic, but that hardly mattered.
My one little complaint would be that there is nowhere to put luggage in the rooms, so you're really living out of your suitcase. Also, be aware that there are no plugs in the rooms so iPhones and kindles need to be charged in reception... we were a little paranoid about leaving our tech there at first, but got used to it after a while.
In short, an absolutely gorgeous and relaxed place to stay in Tulum. Highly recommended!More