Aktun Chen, about 10 minutes inland from Akumal, is one of the few large dry caves on the Riviera Maya that are open to walking visitors rather than divers. Guided tours lead through illuminated chambers of stalactites and stalagmites, ending at a small cenote where you can swim if you have brought a suit.
The same property runs a small jungle zip-line course and a canopy walk through the surrounding forest. It is touristy, but the cave itself is genuinely impressive and the guides are knowledgeable about Mayan history. Good half-day option when the sun gets too strong for the beach.