Try spelunking at the impressive limestone caverns of Ak'tun Kan, which translates from Q'eqchi' Maya as 'Cave of the Serpent.' The cave-keeper provides the authorized interpretation of the weirdly shaped stalagmite and stalactite formations, including the Frozen Falls, the Whale's Tail and the Gate of Heaven, the last within a great hall where bats flutter in the crevices.
The provided helmets have a small light inside them. Explorations take 30 to 45 minutes. It's 2km south of Flores; take a tuk-tuk there (Q15).