There is a beautiful place you can safely spend a couple of nights right behind the dunes at the beach. Once you head from Tulum towards Punto Allen, right when the pavement ends, there is a small entrance station to the Parque Natural. I guess you could drive by without stopping, however, I recommend you stop and pay the small per person, per day entrance fee as well as the 25 Pesos for your vehicle. Afterall those fees are what help proctecting the beautiful natural surroundings, beaches and its wildlife. Count your kilometers from the entrance station on, after 2.7 kilometers respectively 1.7 miles there is a fenced-off area east, to your left, towards the beach. It is marked "propriedad privada" and has two entrances. You can drive in and park/camp there. The area is suitable for any kind of vehicle as the ground is reasonably stable. I think it is perfect for tent camping as well. However there are no services, no toilets (dense bushes near by should solve that problem), no fresh water, no trash cans. Their may or may not be some one to come and collect a small fee, we paid 50 Pesos for 2 nights. The hombre who collected said that was the fee per night but since they did not check for the past couple of days he would only charge for the actual day he came by. More than fair I believe.
The beach is fantastic, no jelly fishes, waves just big enough to have fun and small enough that you can get over them and swim as well. Beware of the current and don't swim far off-shore. I'd believe you could kayak as well if you are experienced enough to get through the initial waves. I did not kayak there.
The place is really easy to find, nevertheless here are the coordinates:
Strong winds make sure you don't have to worry about mosquitos too much. Take a flash light when you go out and about at night. Plenty of fairly large sized spyders camp there next to you. Beautiful colored birds will wake you up with there singing in the morning.