It's hard to believe that Cancun, known to many as the ultimate vacation site, was founded only in the early 1970s. In the "old" days, the pioneer resorts had to compete with established tourist destinations worldwide, and cannily used free vacations as p rizes on television game shows as a means of quick mass merchandising. Those color shots of acres of white sand and tantalizing blue water stretching beyond the ultra luxury of brand-new resort complexes did the trick. Cancun became a favorite of Nord Ame ricanos who wanted state-of-the art accommodations in an unspoiled tropical paradise right next to home.

Cancun is the same going into the '90s. Only more so. Development of new and bigger facilities has been accomplished with today's sensitivity to the environment, and the Mexican Government, neck-deep in negotiations with the United States over the Free Tr ade Zone, is quickly developing a modern tourist infrastructure to support places like Cancun.

Some may scoff at the mention of a resort, which usually means a self-enclosed, hermetically sealed environment totally divorced from its surroundings. In most, you might as well be on the moon as on whatever bit of sand and ocean was used to place the pi lings for the hotel's foundations. But here it is different.

For example, the three hotels run by the Cancun Hotels Cooperative -- the Hotel Cancun Palace, the Hotel Cancun Beach Club, and the new Cancun Puerta al Sol Hotel Suites and Spa, (where I stayed) -- are very much connected to their heritage. Not that thes e aren't fully complete resorts in every way. They are, offering a stunning array of sports, entertainment, dining, and just plain lazing in the sun facilities. They operate on the interesting principle that may be thought of as a land-based cruise ship - - everything from health club to food is included in the price. But the management is also in love with the Yucatan, and makes every effort to get visitors out and about exploring this fascinating piece of real estate. Guests even get a choice of three fr ee tours exploring the socially and historically interesting peninsula.

Geologically, Cancun is a phenomenon. Nobody is quite sure what this L-shaped piece of sand and rock is. It's not an island, although it is called that. It's really a permanent coral-based sandbar with 13 miles of white powder beach separated from the mai nland by two 100-yard channels. The white sand, which comes from the underlying porous limestone, never gets hot in the sun.

Cancun itself offers all one could want in the way of sightseeing, shopping, dining, and nightlife, as well as providing a glimpse into the Yucatan's unique lifestyle and culture.

Dining can be as tame or exotic as you please. Catering to an international crowd that includes families means you can't draw borders on your cuisine. A grilled cheese sandwich, a pizza or burger, and egg roll, even Tex-Mex, are all easy to get. There are restaurants serving French, Italian, German, Chinese, Japanese, Polynesian, Swiss, Argentinian, Lebanese, Spanish, vegetarian, and all-natural health foods. But the fun is in going native. Cancun offers both traditional and regional Mexican cooking of th e kind you can't get in the U.S. Forget the tacos and burritos, and start the day with a breakfast of chilaquiqles. And don't miss a meal of pavo en relleno blanco -- typically Yucatan. What are they? Come to Cancun and find out.

The numerous resorts of Cancun offer evening shows and restaurants featuring Mexican entertainment. A taste of pirate life can be had at the Treasure Island theme park on Isla Mueres. And don't miss its no-holds-barred "Pirate's Night" beach party. But fo r some real adventure, you must visit Cancun's new 6,000-seat bull-ring, destined to become one of the world's great capitals of bullfighting. It is already attracting some of the great Mexican and Spanish bullfighters to its arena.

If Cancun is a relatively new development, the area surrounding it is rich in history. Nearby was an important Mayan site. Periodically, ancient Mayan kings convened on the peninsula's shores to discuss matters of commerce. A few miles from Cancun lie the impressive ruins of Chichen-Itza, one of the great Mayan cities, dating to about 445 C.E. It was abandoned some 800 years ago, its huge temples and stoneworks slowly succumbing to the dense jungle growth and unknown to the outside world until it was redi scovered in the 1880s. Two other important Mayan ruins are close by -- Cobra, dating from the third century, and Tulum, about three centuries later. Tulum, to the south of Cancun, has remnants of some 60 structures, including a watchtower perched on a 40- foot bluff that commands dramatic views of the peninsula's sweeping coastline.

Visitors to Cancun may want to take advantage of two other interesting day trips. Cozumel is conveniently only a ferryboat away. Its name means the Isle of Swallows, and there's plenty of natural splendor on show at the internationally famous Palancara Re ef National Park, second only to Australia's Great Barrier Reef. Cozumel has a long and variegated history. It served as the 15th-century supply depot for the despotic Hernan Cortez. Two centuries later, its coves provided hiding places for pirates Jean L afitte and Sir Henry Morgan. More recently, the island played host to American airmen stationed there during World War II and to oceanographer Jacques Cousteau, who is given credit for publicizing the area's great natural splendor. The sea and reef are al most as famous as Cozumel's native jewelry business. Shoppers should home in on the island's only city, San Miguel, just to look at the beautiful and mysterious rings, bracelets, and pendants that are fashioned out of the reef's rare black coral.

Reservations and tickets for any of these trips and entertainments can be made at any hotel or resort tour desk.

If you're coming to Cancun from the Miami area, you can fly with Iberia, the national airline of Spain. It's a short flight (under two hours), but you'll really feel that your vacation has gotten off to a good start as they pamper you with every attention .

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