I bumped into Ponce de Leo'n the other day, quite by surprise. No one could have been further from my mind, and I didn't immediately recognize him -- dressed as he was in armored vest and plumed hat under a hot afternoon sun. But of course I knew the name.

My childhood schoolbooks had pictured him, as I recall, as a luckless, somewhat comical 16th-century Spanish explorer who had ventured into the wilderness of Florida in a futile search of a "Fountain of Youth" -- a sort of Don Quixote of the swamps, armor and all. But in Old San Juan, the historically intriguing capital of Puerto Rico and the Caribbean's busiest cruise port, he seemed a rather heroic presence, standing -- as he has for a great many years -- in the lovely Plaza de San Jose'.

Juan Ponce de Leo'n, as I subsequently learned, is an honored founding father of Puerto Rico. He was appointed by the Spanish king as the Caribbean island's first governor in 1508, soon after it was discovered by Christopher Columbus, and he is buried in the city. His family's home for more than 250 years, the elegant Casa Blanca, has been exquisitely restored (as has much of Old San Juan) as a museum depicting the life of a prosperous man in the 16th and 17th centuries.

His handsome statue -- I was drawn to it from a distance to find out who it was who warranted such a prestigiously sited pedestal -- commands the entrance to the charming little San Jose' Church, one of the very oldest churches in the Western Hemisphere. It also was the family chapel of Ponce de Leo'n's descendants.

San Juan, now just a couple of decades shy of its 500th birthday, is wonderfully full of the drama of Spain's conquest of the New World, which like the footsteps of Ponce de Leo'n, invites exploring. Perhaps I should have been aware of Ponce de Leo'n's Puerto Rican connections, but the school texts of my day -- and probably some even now -- focused overwhelmingly on the Western Hemisphere's English heritage, all but ignoring Spain's important role. San Juan, to put things in perspective, was founded more than a century before the Pilgrims landed at Plymouth Rock.

Puerto Rico had no natural wealth to lure the Spaniards, but its strategic location on main Caribbean shipping lanes soon transformed San Juan into a fortress city. Its role was to protect Spanish galleons laden with silver and gold taken from Mexico and Peru. A walled city in the Old World tradition, it became one of the strongest bastions in the Spanish American empire. Its two massive forts, El Morro and San Cristo'bal, stand now as impressively majestic as when they were built.

In a way, its location still plays a vital part today in the city's economy. Midway in the long chain of Caribbean islands that stretch south from Florida to South America, San Juan is both an important port of call for many ships sailing out of Florida, and increasingly it is the home port for cruises headed for the distant islands to the south. About a half million cruise passengers visit the city annually, and their number is growing.

The ships, sometimes several of them at a time, snuggle right up into Old San Juan, the city's still-walled, seven-square-block historic area. Passengers who enjoy exploring on foot can do so quite conveniently. As a welcome, the Tourism Information Center at the dock treats everyone to a free rum (Puerto Rican Bacardi) punch and a helpful street map.

Obviously, the shopowners along Calle Fortaleza and Calle Cristo -- two main shopping streets -- prefer that visitors devote their shore time to the fashionable boutiques and crafts shops that are a part of San Juan's appeal. But leave the souvenir buying until later -- after the disembarking crowds have thinned -- because the Old San Juan that awaits is probably the loveliest and most interesting city on any Caribbean cruise itinerary.

Indeed, the old city is a place a historically minded vacationer might fly to for two or three days of sightseeing and dining without any intention of seeking out the island's beach resorts. The seven square blocks house some 20 museums -- including the tiny Pablo Casals Museum on the Plaza de San Jose' honoring the famed Spanish cellist, who lived in Puerto Rico the last 20 years of his life.

The whole historic district, in fact, is a museum. Spared the high-rise office buildings that once threatened it, Old San Juan is now one of the most beautifully preserved and restored of Spain's former colonial cities. Old Spain passed along much of its architectural charm to its former possession: tiny plazas and secluded patios; pastel-shaded townhouses adorned by wrought-iron balconies; a soaring cathedral and narrow, cobblestone streets graced with gorgeously flowering bushes and trees.

Enhancing this beauty is the old city's impressive setting on a small, rock-rimmed islet. Surrounded on three sides by water, the historic area is splendidly detached from the sprawl of modern San Juan, a noisy, traffic-choked metropolis of about 1 million people. Old San Juan has its traffic jams, too, but it still maintains the quiet, relaxed pace of an earlier century. Some streets are restricted to pedestrians only.

This tiny enclave offers fine water views from almost any vantage. The most spectacular seascapes are on the Atlantic side, where the green waves crash against rocky cliffs. From this high point, dominated by the two massive forts, the streets tilt downward to the tranquil waters of San Juan Bay, where the cruise ships dock. The slope is mostly gentle, but occasionally the descent is so steep that a street dead-ends into rough stone steps. Old San Juan's nearest beaches are a 10-minute taxi ride away in the high-rise resort strip of Condado.

Only a few years ago, San Juan's rich heritage was very much in jeopardy. In the '30s and '40s, the old city was little more than a slum. But as the island grew more prosperous, investors looked upon the area as a place to build high-rise office buildings. Ultimately a group of preservationists banded together in 1955 to form the Institute of Puerto Rican Culture to save Old San Juan. In the years since, some 400 structures have been restored, public buildings as well as private residences. Old San Juan has become the "in" place to live for Puerto Rico's yuppies.

"Some people wanted to make Old San Juan into a little New York," recalls Ricardo Alegria, who headed the institute for 15 years and is still a force in the city's preservation. "I said, 'Not even all the people who live in New York like New York.' " Some of San Juan's architects criticized him for "wanting to live in the past" and complained that he was stifling their creativity by fighting modern structures in the old city.

They are more conscious now, he says, of the importance of what the institute has attempted to achieve: "Although we are politically associated with the United States, we want to preserve our own culture." He sees Old San Juan as an example for other Latin American cities whose heritage is being buried beneath steel and concrete.

My starting point for a tour of Old San Juan was not a cruise ship but the distinguished Gran Hotel El Convento, the historic area's only first-class hotel. The structure was built more than 300 years ago as a convent for Carmelite nuns, which is how the hotel got its name. Back in the '50s it almost was bulldozed to make way for a parking garage until rescued by the institute.

It is an imposing structure, built for the centuries with walls three feet thick of handmade clay bricks, and conveniently located in the heart of the old city. Just across Calle Cristo is San Juan Cathedral, where Ponce de Leo'n rests in a marble tomb. And a block down the steep little street called Caleta Las Monjas is San Juan Gate, a historic passageway through the 40-foot-high, 20-foot-wide City Wall. It leads to a small cove where sailing vessels once anchored. Youngsters now splash in the water there.

In earlier centuries, sailors traditionally entered the city through the gate, climbed Caleta Las Monjas past the convent and entered the cathedral to give thanks for a safe arrival.

Much care has been taken to retain the hotel's colonial look. Wooden doors were handmade in Spain, as were the huge chandeliers, which are reproductions of the 17th-century originals. And the guest rooms, which open onto a garden courtyard, were decorated with antique clay tiles, carved wood furnishings, goatskin lamps and handwoven bedspreads. After a couple of nights in these evocative surroundings, you began to think you need a suit of armor to match Ponce de Leo'n's.

There really is no logical best way to explore Old San Juan. It is a city made for wandering. I toured the old forts, both a part of the San Juan National Historic Site, to get a sense of the old city's pivotal role in defending Spain's treasure ships. I walked the wide tops of the old sandstone walls to get the sea views and the cooling breezes. And then I ducked back into the tangle of streets to find an open-air restaurant serving cold Puerto Rican beer and a spicy shrimp stew called asopao. As Puerto Rico's oldest restaurant, La Mallorquina on San Justo Street did quite nicely.

I got a kid's joy out of the stone sentry boxes that jut from the walls of El Morro. They hang out over the wave-splashed rocks, so the views are especially good. The box is only large enough for one person. You can stand there alone pretending, as I did, that you are the guard on duty keeping watch through the slits for any threat from the sea. Sir Francis Drake, the famous English sea captain, attacked with a fleet of 26 vessels in 1595 but failed to take El Morro and sailed away after three days of trying.

El Morro was begun in 1539 to defend against such attacks by sea. San Cristo'bal got underway in 1634 to thwart any land invasion. Both forts look very much the part. They are massive structures, stair-stepping up the cliffs. Hidden stairways, wide ramps and long tunnels lead between the levels. Still thinking like a kid, I figured either would be a marvelous place to play hide and seek.

When your legs tire of climbing, head for the Pablo Casals Museum, a cool respite from the Caribbean sun. The museum details Casals' distinguished musical career with walls of photographs, a display of his awards and his cello encased in glass. In the background, a tape deck plays Casals in concert, and you can request videotapes of past performances at the annual Casals music festival. When you step from the museum, pay a courtesy call on Ponce de Leo'n, who waits just ahead.

Another stop for any visitor should be La Fortaleza, the official home of Puerto Rico's governor. It is a beautiful structure dating from 1540 and generally considered the oldest executive mansion in constant use in this hemisphere. Escorted tours of its elegantly decorated formal rooms are offered weekdays. If you spot a couple of bunnies hopping through the sunken garden as you exit, they are the pets -- so the guide told us -- of the governor's children.

It was at La Fortaleza in 1898 that Spain's flag was lowered and the U.S. flag was raised in its place, the result of this country's victory in the Spanish-American War. Since 1952, when Puerto Rico became a U.S. Commonwealth, both the United States and the Puerto Rican flags are flown over La Fortaleza.

Oh, but now you want to go shopping before your cruise ship whistles its departure. Hold back for just a few minutes more and search out La Plazuela de la Rogativa.

This little plaza -- next to the City Wall near the San Juan Gate -- bears the name of a graceful modern-day statue that commemorates a romantic bit of Old San Juan history. The statue depicts three women and a clergyman who in 1797, so the story goes, were seen by a British fleet besieging the city. The San Juan citizens were taking part in a rogativa, a religious procession. But the British mistook them with their flaming torches for Spanish reinforcements and sailed away.

Shaded by trees and cooled by the breezes from the bay, the plaza is a pleasant place to relax and contemplate what you have seen. The story of La Rogativa may be only a historical curiosity, but it is an appealing one and, for me anyway, it managed to give life to the early residents of Old San Juan who depended upon their massive walls and fortresses for protection. Discovering such anecdotes is part of the fun of exploring Old San Juan, or any city for that matter.

And so is chancing upon an old schoolbook friend such as Ponce de Leo'n.

WAYS & MEANS

GETTING THERE: Both American and Eastern fly nonstop daily to San Juan from Baltimore/Washington International Airport. An unrestricted round-trip ticket currently is being offered at $384 on weekdays (Monday-Thursday) and $404 on weekends. CRUISES: Among the cruise lines operating out of San Juan are Carnival Cruise Lines, Chandris Fantasy Cruises, Costa Cruises, Exploration Cruise Lines, Norwegian Cruise Lines, Princess Cruises, Royal Caribbean Cruise Line and Royal Cruise Line. For specific details about these cruise lines or other lines that call at San Juan, consult a travel agent. WHERE TO STAY: San Juan has a number of fine, beach-front hotels, but the only luxury hotel in the historic district is the excellent Gran Hotel El Convento. A former convent, it has been restored with great care to retain its Spanish colonial style. The large interior courtyard has a bar, restaurant and swimming pool. The price for a room for two is $125 a night. The hotel has just been placed under the management of the Ramada Inn chain. For information: Ramada Inn, (800) 228-2828 or Gran Hotel El Convento, P.O. Box 1048, San Juan, Puerto Rico 00902, (809) 723-9020. INFORMATION: Puerto Rico Tourism Co., 1290 Avenue of the Americas, New York, N.Y. 10104, (800) 223-6530 or (212) 541-6630. -- James T. Yenckel