Arroyo is a small seaside town on Puerto Rico's south coast, just east of Guayama. It is not a polished resort town, and that is part of its appeal. The waterfront is local, the beaches are low-key, and the town's history turns up in odd places: an old customs house, sugar ruins, and a monument to Samuel Morse, who helped introduce the telegraph to Puerto Rico here in the 1850s.
The best short visit links the town centre, Old U.S. Customs House, Malecon Paseo Las Americas, and the beaches near Punta Guilarte. Punta de las Figuras Lighthouse is the most atmospheric sight, a Spanish colonial lighthouse near the shore, damaged and not open inside but still worth seeing from outside.
Arroyo works best if you are already exploring southern Puerto Rico and want a quiet coastal day with history around the edges. It is not worth skipping Ponce for, but it is a good antidote to busier beach towns.