The series of waterfalls and crystalline blue pools at Agua Azul is breathtaking, especially during the dry season (from about November through March), as wet-season waters are often churned up and brown with mud. You can swim in a series of interconnected pools.
If the single cascade at nearby Misol-Há is less grandiose than the series of falls and pools at Agua Azul, it's no less amazing. You can swim in the pool formed by the 100-foot cascade, or explore behind the falls, where a cave leads to a subterranean pool. (If there's a guide with flashlight in hand to help you, tip him $1 or so.)