San Diego

Languishing lazily by the Pacific Ocean, somewhere between Los Angeles and Tijuana, lies one of the fastest-growing cities in America. Imagine a town where the daytime temperature always hovers around 70 degrees F., where nights are always cool and comfortable, where the shore is lined with white-sand beaches backed up by luxury hotels and charming bed-and-breakfast pensions, where wildlife reigns supreme in super zoos and theme parks - and which lies only 15 miles away from colorful Mexico.

San Diego is now the country's eighth-largest city and the second-largest in California. No wonder. I found it to be one of the most livable - and vacationable - cities I have ever visited in the United States. And for good reason - a relaxed pace, 70 miles of marvelous beachfront, comfortable living spaces, a wealth of recreational facilities, a growing number of cultural opportunities, and many Navy-oriented industries.

I meandered down from Los Angeles by Amtrak. The 21/2-hour, $16.75 one-way ride (you might want to take the 25-minute flight back) started from L.A.'s mission-style Union Station, where in bygone days newly signed Hollywood starlets used to arrive from the East Coast to be met by bored press agents. The first hour of this early morning coastal train ride (the dining car serves stand-up or take-to-your-seat breakfasts) moves passengers through the ugly industrial outskirts of Los Angeles. But soon the ugliness gives way to such charming towns as San Juan Capistrano, the cliffs and beaches of ex-President Nixon's San Clemente, and the breakfast-on-the-beach circles at La Jolla. The train hugs the coast, and as it nears San Diego, it chugs over the rolling San Diego hills.

As I left the train (it stops on a track far out from the platform), a blonde surfer-type redcap came dashing over to hoist my bags onto her cart. In seconds I was in a taxi on my way to a turreted, cupola-capped gingerbread castle on San Diego's Coronado Peninsula: the famous Victorian Del Coronado Hotel, known to locals as ''the Del.''

Opened in 1888, the Del Coronado has been designated a National Historical Landmark by the US Department of the Interior. Although much of it has been modernized and updated with private bathrooms, glassed-in beach-front towers, and an Olympic-size swimming pool, its public rooms remain essentially as they were when it served as the summer home of some of America's wealthiest families. I remember scenes from Marilyn Monroe's picture ''Some Like It Hot,'' which was shot at the Del.

Prices range from $68 single or double for a room without a view in the old main building to $175 for an ocean-front, poolside room, with lots of prices in between for varying degrees of comfort and view. I advise looking over the room before accepting it - your decision should depend to a great extent upon how much time you will be spending in the room and how much time you will be spending in the Del's wood-paneled lobby-lounge, its many restaurants and ocean-view cafe, its chic shopping mall, its tennis courts and swimming pool, and its marvelous surf beach.

Since I was in San Diego for only two days, I decided to make the hotel my dining headquarters. Besides a low-priced deli in the mall, there is a gourmet restaurant in the hotel, the Prince of Wales Grille, which cost me around $30 for a dinner of prosciutto and melon, filet mignon, and chocolate mousse. However, I much preferred the ambiance of the Crown Room, an enormous pegged sugar-pine-ceiling room with no visible supports capable of seating 1,000 people. A $17 full-course rack-of-lamb dinner proved to be a bargain, if not exactly of gourmet quality.

But my greatest pleasure in the hotel came from an old Victorian habit of mine - leisurely lounging in the huge wood-paneled lobby, watching the old caged elevator ascend and descend, imagining the comings and goings of the Vanderbilts , the Astors, and the Tiffanys who used to vacation here. Not to mention the Duke of Wales, who reportedly first met Wallis Simpson at the Del.

A short taxi ride (be forewarned: since the Del is on a peninsula and there is a $1.20 bridge toll each time you enter, taxi rides to the center of town may be as high as $10) from the Del Coronado, located in Balboa Park, is the world-famous San Diego Zoo. One visit, and you will understand why its reputation is worldwide. One of the world's best and best-kept collections of animals, this 100-acre menagerie features everything from koalas to Komodo dragons. And, because of San Diego's subtropical climate, the zoo is also a botanical garden filled with exotic, colorful plants.

Entrance fee is $4.95, which entitles you to everything but the bus tour ($2. 25) and the spectacular Skyfari (75 cents), which affords visitors a panoramic view of the whole complex from a height of 170 feet. If you are wary of ski lifts, you may shy away from this, but in any event, take the bus tour. Be sure to stay in the lower section of the bus, unless you are willing to sit in the sun.

The Sea World theme park in San Diego is the original of the several Sea Worlds around the country. It is located a short bus trip away from the Del on Mission Bay. The $10.95 admission price entitles you to see all the major exhibits and shows. There's also a sky tower that takes you up 320 feet for a panoramic view of the park as well as San Diego itself. Then there is the Atlantis Skyride, a cable car that makes a half-mile trip across Mission Bay, and the hydrofoil boats that take passengers on tours of Mission Bay. The three rides are $1.25 each.

Besides the rides, there are many places to sit down and take a break - something I like to do - at Sea World. Eating places, of course - at surprisingly reasonable prices. And shows. Whatever you do, don't miss one of the most exciting shows of all - Shamu, the killer whale, performing unbelievable tricks. There's also a seal and otter show, a dolphin show, and a pearl-diving exhibition. You can sit at most of these.

But you can't sit at one of the most intriguing exhibitions in the park - the Penguin Encounter. In a simulated Antarctic environment, more than 300 penguins slide across the ice, flap their flippers, stand around looking at you as you walk through the 5,000-square-foot area. Much scientific information is dispensed on TV monitors. There's no place to sit, but nobody stopped me from leaning heavily on the rails.

In the heart of Balboa Park, the Simon Edison Center for the Performing Arts has brought a theatrical renaissance to San Diego. The Old Globe Theater group, a year-round, three-theater complex, now houses a Shakespeare Festival, and two outdoor theaters in the same compound do other plays, as well as Shakespeare.

I took a bus from Coronado to Balboa Park, where thousands of people were pouring into the theaters to see ''Twelfth Night,'' ''Henry IV,'' and ''Talley's Folley.'' I chose to see Marsha Mason in a delightful version of ''Twelfth Night ,'' directed by Jack O'Brien. It was a highly enjoyable evening, filled with authentic Elizabethan songs. The steep ravines, eucalyptus, and bougainvillea of the park turned the complex into a kind of tropical amphitheater. The Old Globe itself is only a partial replica of the original Globe - what was originally a pit where spectators stood now houses comfortable seats.

Tijuana in Baja California is only 15 miles, less than one-half hour, away from the center of San Diego. Nobody should miss it. Especially now that you can go there by trolley.

One-way fare is $1, and the Amtrak trolley will let you off right at the San Ysidro border. Despite its sleazy reputation, Tijuana is beginning to earn a reputation as a new cultural center. For those Americans who have seen only the nightspots and the souklike shopping pasajes, the new Tijuana deserves another look.

First, there's the fun of shopping at the International Trade Center, with its native crafts at reasonable prices. Then, there's the new Tijuana Cultural Center, conceived by the Mexican government to ''further mutual understanding between the people of the US and Mexico.'' Since so many Americans in the past have gone no farther south in Mexico than Tijuana, authorities have decided that it is necessary that somewhere in Tijuana there be an exhibition of the cultural and geographical glories of Mexico, as well as the shopping spots, bullfights, and special entertainments for which it has become noted.

Designed by Pedro Ramirez-Vasquez and Manuel Rosen Morrison, the spectacularly architected center may become as much a tourist highlight in Mexico as the famous Anthropological Museum in Mexico City. The complex encompasses a concert hall, the museum, an omnitheater, and a restaurant. The day I was there, the Anthropological Museum in Mexico City had mounted a superb exhibition in the museum. And in the 360-degree theater, I saw an excellent film on the history of Mexico, as well as what amounted to a spectacular helicopter ride over the major architectural, archeological, and geographical sites of Mexico.

Flying back to Los Angeles on the 30-minute trip from San Diego, as the plane rose over the city, I looked down and could make out the distinctive cupola of the Del Coronado. As the plane soared higher, I could see most of the 70 miles of beaches in the San Diego area - and further to the south, Tijuana, too. What was once the exclusive habitat of the Vanderbilts and Astors has now become an ideal resort city for everybody, I thought. Practical information

Besides the Del Coronado, San Diego has a wide selection of much-less-expensive hotels, motels, and bed-and-breakfast pensions. For further information on hotels, restaurants, and all the sightseeing opportunities mentioned, as well as others too numerous to include in this article, write to the San Diego Convention and Visitors Bureau, 1200 Third Avenue, Suite 824, San Diego, Calif. 92101.