WHAT IS 100 miles long, 35 miles wide, more than 4,000 feet high and covered with sunshine? Puerto Rico, that's what, if you exclude the rain forest.

But the easternmost island of the Greater Antilles -- a 3 1/2-hour flight from Washington -- is more than sunny beaches, ideal temperatures and gambling casinos. A trip there during the cold, dreary months of winter proves it to be a badly needed tonic. Not only does the temperature stay in the 70s and 80s, but you are reassured that rain can occur without the numbing cold and dreadful depression that so often accompanies it in Washington.

And since the island is a Commonwealth of the United States, one finds some of the familiarity of home. The currency is no problem and English is spoken almost everywhere, although the predominant language is Spanish.

Caribbean islands are delicately balanced tourist havens, and sometimes their seasons in the sun can be interrupted by internal probems or attitudes, real or faniced. Puerto Rico, having recovered from those days when visitor complaints were frequent, is now very much in style.

This is not to say that Puerto Rico is the ideal place for "getting away." There are 3.3 million people crowed onto the island. Traffic in the metropolitan area around San Juan, where more than half a million live, is as slow as the native music is fast. Horn blowing must be a required course in every school.

A visitor could spend a week in Old San Juan alone. This beautiful walled city is a complex of narrow, charming streets lined with ships, galleries and restaurants. The focal point is Fort El Morro at the western tip -- built in the 16th centruy by Spain to protect San Juan Bay. The massive fortifications are on six levels and provide some spectacular views of the ocean. The National Park Service recently outlined a $25.4-million restoration project to prevent erosion at the fortress.

El Morro is part of the San Juan Historic Site along with San Cristobal, a fort constructed to protect the city from land attack. San Cristobal is in five independent units and rises 150 feet above the sea.

One of the more romantic spots in Old San Juan is the charming El Convento Hotel. A buffet is served in the hotel's patio restaurant, or in the charming, converted chapel where a flamenco show is available most nights. And no visit to the old city would be complete without a tour of the cathedral or a walk to the dock area among the many cruise ships.

El Cristo Chapel, built in 1753 and said to be one of the smallest in the world, is situated atop a precipice from which a young horseman plunged during a race but miraculously lived. The chapel was built to close the street.

The plaza in the old city is one of the largest on the island and a visitor can often find a show there by native musicians and dancers. A plaza and adjacent church are to be found in every town in Puerto Rico and are generally the center of social activities.

In Old San Juan a visitor would be remiss not to pick up some of the native crafts and sample the delightful island cooking -- including the spicy rice dishes and the variety of offerings of both ripe and raw plantain. These can be found in most restaurants on the island, but even if the language problem is heightened it is recommended that the visitor frequent the small restaurants preferred by the islanders. Here the food tends to be more authentic.

Part of any vacation should be the adventure of experiencing something new, tasting new dishes, observing a different culture, savoring a contrasting lifestyle. This can be done easily in Puerto Rico. But it is possible to spend a vacation on the island eating only familiar American food, hearing only English and seeing none of the sights. One American recently was overheard to comment in an island restaurant, "I don't want any of that native food, just give me a steak."

And a guest at a hotel along the Ashford Avenue strip in Condado recently told an inquiring reporter that he and his wife and kids come every year to San Juan, eat at the hotel, lie on the sand all day on the hotel's beach and go to the casino at night -- no sightseeing, no experiencing the rest of the island.

Most of the tourist area is situated along the Atlantic beaches near San Juan, but there are better beaches to be found in several locations just a short drive out of the city, such as Luquillo with its broad expanse of sand and many tall coconut palm trees. Also available are facilities for side trips, many of them only for the day, to some of the nearby islands in the Lesser Antilles group.

A variety of activities are available everywhere, including surfing, snorking, diving, water skiing, tennis and golf. Horse racing is available at night at the attractive New El Commandante race track just outside the city, and during the winter months sports fans can take in Winter League baseball at Hiram Bithorn Stadium in San Juan and in other towns on the island. The cockfighting season runs from Nov. 1 to Aug. 31.

There is a variety of cultural activity centered on the University of Puerto Rico's sprawling campus, and many downtown hotels present shows, some of them similar in style to those at Las Vegas hotels.

But Puerto Rico is not just San Juan and its metropolitan area. It is dense forests, majestic mountains and cactus-spotted deserts as well -- "out on the island," as the natives describe anywhere beyond the metropolitan area.

A fascianting 25-mile trip east of the city will take a visitor to El Yunque, the affectionate term used to describe generally the 28,000-acre Caribbean National Forest, whose main attraction is the rain forest. A twisting drive up the mountain carries visitors through a variety of more than 600 trees, only six of them native to the states. Various parking areas are available along the way, permitting travelers to view the various waterfalls and marvel at the scenery, which includes the Atlantic Ocean to the north.

Hiking trails are numerous and much wildlife is visible. And you may be able to hear the two-note call of the coqui, a tiny tree frog, which scientsts have been unable to raise anywhere but in Puerto Rico. To cap off a trip to El Yunque, a visitor can picnic tables scattered through the palms or like through the forest to a restaurant near the peak where native food is served.

Another day's trip can be south across the island to Ponce, the island's second largest city, a trip of about two hours by car. The scenery is almost worth the trip, particularly when the traveler suddenly tops a mountain to see the Caribbean below.

At Ponce, a visit to the Museum of Art, housed in a building designed by Edward Durell Stone and sometimes called the Parthenon of the Caribbean, is in order. The collection is designed to represent both American and Euripean art, although the museum is only a few years old. The main plaza at Ponce is dominated by the Cathedral of Our Lady of Guadeluped and the firehouse, which is painted red, black, green and yellow and serves as the city's tourist office.

Much of the countryside is worth savoring. The recent upswing in tourist traffic to the island makes a respite from San Juan's teeming souvenir shops not only attractive, but almost necessary.

This recent increase in visitors threatens to become a flood as airline deregulation and increased competition continue to send air fares lower. Most of the tourists continue to be Americans, Venezuelans, Spaniards, Canadians, Cubans and Puerto Ricans living in the States. But traffic is coming more and more from such places as Scandinavia and other colder points.

Although lounging around continues to be the main activity of tourists, more and more visitors are attempting to get to know the island, to visit with Puerto Ricans in their homes, to exchange ideas and to take advantage of the islanders' greeting:

"Welcome. This is your home."