Halfway across Isla Colón [1] on the road to Boca del Drago [2] 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.
Links:
[1] http://www.moon.com/destinations/panama/bocas-del-toro/isla-colon
[2] http://www.moon.com/destinations/panama/bocas-del-toro/isla-colon/sights/boca-del-drago