Halfway across Isla Colón  on the road to Boca del Drago  is La Gruta (literally, “the cavern”), home to zillions of bats. It’s at the little settlement of Colonia Santeña, near the Coca-Cola sign. A little store there sells drinks.
If your idea of fun is to wade through guano in a claustrophobic cave and inhale disease-carrying particulates, then by all means walk through the thing. The rest of us will be happy with a snapshot of the highly photogenic entrance to the cave, a green grotto with a little statue of the Virgin standing guard.
Round-trip taxi fare from Bocas town is US$10. The Boca del Drago bus (Transporte Boca del Drago, cell 6607-7681) can drop you there, but check the return schedule to avoid getting stranded.