Semuc Champey is the anchor sight for Lanquín: a natural limestone bridge over the Cahabón River, with clear turquoise pools stepping down through the forest above the river's hidden channel. You can swim in the pools, climb to the mirador for the classic view, and watch the brown river disappear under the stone before it reappears downstream.
The site is close in distance but slow by road, so start early and do not treat it as a casual walk from town. It is worth the trouble. Few places in Guatemala feel so physically satisfying after a sweaty ride: cold water, green forest, and the strange sight of a river running underneath your feet.