Puerto Rico
Puerto Rico, at 3,515 square miles, is the smallest of the four islands that comprise the Greater Antilles. In 1508, Juan Ponce de León explored the island, and he became its first governor.
Puerto Rico provides rich opportunities to sail, surf, snorkel, scuba dive, hike, horseback ride and sun bathe. You can also explore the great outdoors in the forests, wildlife refuge, rainforest, beaches, and adjacent islands. For more ideas, see The 15 Best Things to Do in Puerto Rico (Tripadvisor) and 29 Best Things to do in Puerto Rico (CS Ginger Travel).
San Juan
San Juan is located on the north coast of the island of Puerto Rico, is the capital and the largest city. It was founded in 1521 and is one of the oldest European-established cities in the Americas. San Juan is a major port, financial hub, and cultural center and includes Old San Juan, Condado and Isla Verde districts.
Old San Juan.
I will let this website speak for itself.
There are a number of walking tours of Old San Juan.
Forts
Ponce
Ponce is located on the southern coast, and is the second oldest city. It is an important port and cultural center with neoclassical architecture.
Ponce Art Museum (Museo de Arte de Ponce)
Centro Ceremonial Indigena de Tibes
Near Ponce, this is a Taíno/Arawak site with ceremonial plazas and artifacts. It is older than Caguana, dating to 400-1000 CE.
Utuado
Caguana Ceremonial Indigena de Cagüana (Parque Ceremonial Indigena de Cagüana in Spanish) (1200-1500 CE) is near the town of Utuado in the central mountains of Puerto Rico’s interior. The setting is a broad, grassy valley ringed by forested limestone hills. It features ten rectangular and oval plazas (called batelyes) outlined with upright carved stones, some more than three feet tall. The stones are decorated with petroglyphs of Taíno deities, faces, and solar or spiral motifs. This is the most important Taino archaeological site in the Caribbean.