Río Secreto

About a kilometre of half-flooded cave, walked, waded and floated in a wetsuit and life jacket with a guide and a small group. The stalactites, columns and crystal formations are tens of thousands of years old, and the swimming parts are short and shallow: you only need to be able to float. The circuit takes around two hours.

Everyone showers before going in, to keep lotion, make-up and hair gel out of the water, and no phones or cameras are allowed underground; a staff photographer sells the pictures afterwards. The minimum age is four. Skip it if you are claustrophobic, pregnant or find uneven walking hard. Groups are capped, so book ahead, and some packages include transport from Cancún or the Riviera Maya.