Old San Juan is an historic site that should be included in the itinerary of almost every visitor to Puerto Rico.  The old Spanish town, founded in 1521, occupies the west end of a small island at the mouth of San Juan Harbor.  It is bordered on the north by the Atlantic ocean and by the harbor on its south and west.  The town comprises buildings both old and new, though the heart of Old San Juan has been almost entirely restored to the look of centuries past. 

The town of San Juan (originally named Puerto Rico, or "rich port") was begun as an enclave for local settlers to defend themselves from the attacks of Carib Indians.  The Casa Blanca, home of the family of Juan Ponce de Leon, the colonizer and first governor of Puerto Rico, was an early fortified site in the city.  Later, in 1540, a small fort called La Fortaleza was completed.  La Fortaleza, now the governor's mansion, overlooks the anchorage in the harbor.  At some time following the colonization of the island first known as San Juan, the king of Spain began referring to the island as Puerto Rico; apparently no one wanted to correct him.  The names of the town and the island were switched.

Note: Old San Juan is not very handicapped-accessible.  The sidewalks are very narrow, and the streets are paved with cobblestone.  There are ramps at some corners, but not many.  The forts have wider walkways and ramps, even an elevator, and can be accessed directly from the road or parking without having to deal with the sidewalks of San Juan.

El Morro and San Cristobal

In the 1540s work was begun on the fortifications that would eventually become the Castillo de San Felipe del Morro, or "Castle of Saint Phillip of the Headlands."  This large, imposing fort is probably the most striking of the defenses of San Juan, and the chief attraction of Old San Juan.  Like its sibling fort, San Cristobal, El Morro has grown and been strengthened over the centuries, and eventually became interlinked with a wall surrounding the entire old town.  The forts were built to defend the town and harbor of San Juan from attacks by pirates and invasions by the French, English and Dutch.  While Puerto Rico itself had little to offer in terms of gold or gems, San Juan harbor was an important strategic base for Spain to keep out of the hands of her European enemies.  The biennial treasure convoys from Mexico and South America, which Spain counted on to finance warfighting in Europe, would meet in Havana then sail around Florida and across the Atlantic.  If Puerto Rico fell into the hands of an enemy country, the treasure convoy would have been doomed.

The island of Puerto Rico was ceded to the United States following the Spanish-American war, in 1898.  As the US entered World War II, additional observation towers and bunkers were built into the fortifications of El Morro and San Cristobal, as well as a decontamination chamber in the case of chemical attack.  A warren of tunnels (closed to the public) connects the two forts.

Some Other Places of Interest in Old San Juan:

Old San Juan is best enjoyed at a stroll, looking in through open doors at gorgeous, tiled interior patios and graceful archways; watching children feed the flocks of pigeons in the plazas and parks; stopping off for a "piragua" (a handmade shaved-ice snowcone) in a shady nook, ducking into a souvenir shop for trinkets, or a dark bar for a pina colada or a cold beer.  Old San Juan deserves somewhere between half a day and a full day, depending on one's interests.  That said, here are some of the places in Old San Juan which many tourists enjoy visiting:

Casa Blanca - Longtime home of the family of Juan Ponce de Leon, now a museum.  Call ahead for schedule.

San Juan Cathedral - Large church built in 1540 in central San Juan, and burial place of Ponce de Leon.  Kitty-corner from El Convento, a former convent now restored into a luxury hotel.

La Fortaleza - The governor's mansion.

The San Juan Gate - The traditional entrance to Old San Juan, before the city outgrew its walls.

Museum of the Americas - Museum focusing on folk art, located inside the old Ballaja barracks.

Plaza las Armas - A large plaza in central Old San Juan with kiosks and street vendors.  Often occupied on weekends by arts festivals, musicians and the like.

Museo del Nino - A children's museum, best suited for younger children (toddler to 6? 8?).  Located across the street and a small park from San Juan Cathedral.

Some Noted* Restaurants in Old San Juan:

Barra China - Purportedly the restaurant where the pina colada was invented.  They do serve a good mofongo.
Parrot Club, Aqua Viva, and Tantra - Troika of the trendiest places to eat.
Cafe Berlin - Impressive vegetarian menu.
El Patio de Sam - An inviting place to grab lunch after walking back from El Morro.
La Bonbonera - Patisserie/restaurant favored by the power elite from the Capitol and La Fortaleza.  Amazing pastries.

*Not necessarily good, just noted.

Also Worth Seeing:

Especially if you have small children, the park/beach complex of Luis Munoz Rivera Park and El Escambron beach, on the Atlantic near Old San Juan are a good place to visit. 

The park, built by Chicago architects at the turn of the 20th century, is reminiscent of parks in Paris or Barcelona and exquisitely gardened and landscaped, and includes good playgrounds for both toddlers and older kids.  A raised walkway leads from the playground over to the beach, where children can hunt for hermit crabs in the rocky pools over on the east end of the beach.

Plaza Las Americas, in nearby Hato Rey, is billed as the largest shopping mall in Latin America; it's certainly big enough.  The mall boasts just over one bazillion stores, as well as a 16-screen movie theatre, a clubbed-out bowling alley, a Borders bookstore, and a number of upscale restaurants.  If you miss hustle, bustle, and crass commercialism, this place should give you a fix until you get home.

Any true foodie should try to make a visit to the Plaza del Mercado (basically a farmers' market) of Rio Piedras.  The stalls are loaded with local produce, much of it organic, and most of it unavailable outside of the tropics.  Soursop, canepa, mango, papaya, acerola, avocado, and many others may be found here.  There are also a number of vendors of cooked food; rice and beans, bacalao (salt cod), lechon, yuca, empanadas, the list goes on.

The Children's Science Park in Bayamon is a fun place for children, with a number of tiny museums, a small zoo, ponds, paddleboats, and lots of trees.