Isla Caja de Muertos is a small uninhabited island five miles south of Ponce [1] accessible by ferry (Sat.–Sun. and holidays 9 a.m.–5 p.m.) from La Guancha in Ponce [2]. There are hiking trails through the island’s interior, which is a semiarid forest containing a variety of cactus. There are also a cave to explore, a lighthouse built in 1887, and a one-of-a-kind underwater snorkeling trail.
Links:
[1] http://www.moon.com/destinations/puerto-rico/south-coast/ponce
[2] http://www.moon.com/destinations/puerto-rico/south-coast/ponce/sights/paseo-tablado-la-guancha