X'cacel beach, on the Mayan Riviera coast, is one of the best kept secret beaches on the Yucatan peninsula. In between Puerto Aventuras and Tulum, this protected area is home to a turtle protection program. In spring and summer, you can see baby turtles hatching, and making their way back to the ocean. There are no restaurants or bars here, so bring your own picnic and plenty of water. The local caretaker will get you fresh coconut if you ask nicely. There is plenty of shade under the palm trees, and a freshwater cenote, or sinkhole, is just a few steps from the beach, in the jungle.
Further down the peninsula, closer to the Belize border, is Xcalak and its renowned white-sand beaches. This area of the Costa Maya is remote and not yet a main tourist destination. You can dive and snorkel on part of the world's second longest barrier reef, go fly fishing or kayaking, or just laze around on the soft white sand. There are no gift shops or large hotels, and even finding a telephone can sometimes be a challenge. Some small, comfortable guest houses are available.
Also slightly off the beaten track, Puerto Escondido, or "hidden port" in Spanish, is just south of Oaxaca. It is not the easiest of places to get to -- there is a small airport in the vicinity, but flights are not always reliable and access is generally cheaper by car, or bus. Here you will find several beaches, with some of the best surfing in the region, fishermen bringing in their catch to sell at the market, and beachfront "palapas" serving grilled fish and local delicacies. There are some small, reasonably priced hotels and a couple of four-star all-inclusive resorts.