Costa Rica wants you to come on an eight-day, seven-night jet charter trip with hotel, breakfast and free tennis included for $339. The country's No.1 export is coffee ("happiest harvest in years," according to a recent edition of the Tico Times), but the little nation is now eyeing another cash crop - tourism.

The Ticos, as Costa Ricans are known, have launched a $1-million promotion campaign with full-page ads in U.S. newspapers and magazines. Plans are coordinated by the Tourist Bureau with principal funding from the Central Bank of Costa Rica. It is hoped that the charter idea will bring an extra 4,500 visitors to the "Switzerland of Central America" this year.

Charter arrangements for the weekly trips are being handled by Nationwide Tours of Mellville, N.Y., with jet flights by Pan Am from Boston, New York, Philadelphia, Washington and some mid-and far-western cities. From Miami, flights are via LASCA, Costa Rica's national airline. Pan Am also offers the only scheduled service from Washington to San Jose with departures five days a week. There is one stop in Guatemala City.

In addition, the Maritz Company of St. Louis has started a program of incentive trips to be underwritten by large U.S. firms, whereby workers will receive a trip as a bonus for top performance. The "Tour Plan of Richmond" will bring in about 1,800 em ployees of Reynolds Company, between Jan. 27 and March 31, as part of another industrial motivational scheme.

If you don't want the charter package, Costa Rica would like to count you among the 297,000 tourists who visit each year for an average two-week stay. And beyond that, if you are retired on a pension or have a private income of at least $300 per month, you are invited to come and live as a resident pensionado and enjoy numerous tax-free advantages.

Costa Rica is only a little bigger in area than New Jersey, with a population of 1.9 million. One of Latin America's oldest established republics, it is headed by a president chosen every four years who is not eligible for reelection. The army was abolished in 1948. A Civil Guard performs both police and related duties.

Volcanic mountains, both dormant and active, are green and brethtakingly beautiful around the capital city of San Jose, where a gentle and constant breeze blows from the east. Low temperature averages 58 degrees Farebheit, high is 80, and the sun always rises and sets within minutes of 5:30 a.m. and p.m. At 3,750 feet altitude and 9 degrees latitude, thesun is customarily quite warm - and dangerous to the sun bather who accidentally falls asleep. May through October is the rainy season, but "mostly after lunch" the Ticos say. Local tour books call the climate eternal spring; it's like a cool Canadian summer. By whatever name the pure air and attractive weather offer an ideal breather from city pollution.

Mountains, wild flowers, butterflies and cattle, both dairy and beef, are reasons for likening Switzerland to Costa Rica. Unlike the Swiss, however, the Ticos are neither great hoteliers nor great restaurateurs. They are a neat, tidy, easygoing people. Their own ad campaign copy refers to themselves as "friendly by nature" and invites visitors to "experience true democracy, peace and stability."

Friendliness is the magic word for traveling in Costa Rica. The supersophisticated, spoiled tourist may need a certain amount of tolerance. For example, there is an almost total lack of public toilets in San Jose; domestic plane schedules are erratic; trains are slow and dirty; cabs appear to charge what the traffic will bear. It takes patience to wait in line at the airport as the man behind the counter attaches by hand a dozen or more stamps to each tourist card to show the departure tax has been paid.

As Bernal Zamora, assistant chief of the Tourist Bureau, explained recently: "We are building for tourist needs and improving trains to both coasts with air-conditioning and bar service. All our tour guides are now licensed. Eventually we hope to attract U.S. investors by offering a tax exemption to those interested in construction, expansion or installation of works directly of benefit to the tourist industry."

Downtown San Jose is crowded, noisy and provincial - like one big block party. There are several good shops with local paintings and craftwork for sale. A fine collection of Central American antique pieces is on view in the Banco Central's Gold Museum, but the National Museum is tired and dirty without much to show. Costa Rica, unlike her neighbors, reflects little Indian influence. Few natives were found when the Spanish conquered the area between 1561 and 1573.

The jewel of San Jose is the National Theater. A miniature of the opera houses of Paris and Milan, it is full of marble, gilt and many rich woods and has been the pride of every Costa Rican since it was built by public subscription in 1890 and opened in 1897. Unfortunately, it is not used enough. The first and only performance of a colorful and spirited folkloric show was in December, but Zamora says it will be repeated regularly. The National Symphony of Costa Rica will open its season March 17, with pianist Byron Janis under the baton of music director Gerald Brown.

Avoid top-listed restaurants; they are gourmet priced and offer a middle-class meal. Small places serving simple, tasty Mexican, Italian and Latin food are good buys. The Cariari and Irazu hotels are large with convention facilities and are being used for patrons of the charter trips. There are also two or three small first-class hotels.

One-day trips from San Jose include visits to the volcanoes of Poas and Irazu, both spectacular. A tour of the old capital of Cartago and the valley town of Orosi, as well as the farm towns of Alajuela and Heredia, can be made in one or several days. Oxcarts, still the chief farm vehicle, are made in Sarchi, which is another one-day trip. Here the bright painted carts are available packaged in cartons to take with you and put together at home either as a bar, table, storage chest or cart. Trips to each coast can be made in one day. The journey to Limon on the Atlantic-Caribbean cosast is a 12-hour day, if you are lucky.Figure on 14 to 16 hours realistically. One flies down by plane at 6 a.m. and returns by jungle train, a seven-to-nine hour trip. Limon is the main commercial port on the Gulf coast, populated mostly by Caribbean blacks. It is a pleasant surprise to hear the soft accents and calypso music. Here Columbus, on his fourth, and last trip to the new world, spotted the island of Uvita in Limon harbor and named the place the "rich coast".

The train journey is 120 miles long with 52 stops at plantation villages. Kids swing on and off the caboose. With box lunch (beer if you are lucky) you lean out of the window of one of the original carriages on a narrow gauge railroad built by the United Fruit Company and inaugurated in 1877. The train winds and pulls, stops and shudders through hot, humid forests with endless banana trees, cocoabean farms, and empty stretches of Atlantic beach. The train crests the summit at 5,000 feet into clear, chilly mountains with coffee plants and waterfalls, here and there crossing over steep gorges a thousand feet below.

It takes a lot of gung-ho curiosity to make this tirp. In the words of a well travelled Californian, "When it's all slicked up with iced martinis and icy air, the tourists will never know why they did it."

Less dramatic (and less evenful) is the 68-mile run to the port of Puntarenes on the Pacific coast, by plane, train, bus or car. The beach is polluted by commercial shipping in the port, but fishing is good and charter boats ar available. The air is hot and heavy with little wind. Several nearby islands offer camping and hotels, in addition to the town's two or three hotels.

To really know the contrasts of Costa Rica, one must go along the Nicoya peninsula to the open surf beaches in Gunacaste also on the Pacific side. Here it is wild, hot (88 degrees in the shade), dry, windy (gusts of up 22 miles per hour), unpeopled and undeveloped, except for scattered horse and cattle ranches and vegetable farms near the inland rivers. There are 12 beaches with six or seven hotels in al.

Roads are washboard. Is not possible to drive directly from beach to beach. One must go to and from the main town of Santa Cruz, each beach road and spoke of the hub, where one plane a day comes from San Jose.

The Guanacaste beaches are formed by volcanic rock with gray-black sand that looks dirty but is cleaned by sun and tide. The ocean colors are of the romanctic blue-green found in songs and island reefs. Pictures of the moon landscapes come to mind while fishing from rocks for red snapper or grouper; diving for langouste, or walking five or six miles in solitude with only pelicans to talk to. At night giant turtles come ashore to lay their eggs. Next morning their paths look like jeep tracks.

In the jungles and gulleys behind the beaches, "honker" monkeys leap in the trees. These three-to-four-foot-long creatures bark like sick donkeys. Clear your mind, refresh your spirit in Guanacaste but also be prepared for ants, scorpions, grasshoppers, lizards, iguanas, rabbits, glorious tropical flowers, beach grasses, vivid sea shells, and giant dirftwood tree trunks that lie upon the sand like playful porpoises.

The most luxurious hotel in Guanacaste is Tamarindo at Tamarindo Beach - air-conditioned, with swimming pool, 60 rooms and continental restaurant. More beautiful, less plush, is the Antumalal at Junquillal Beach with accomodations for only 20 guests and family-style meals.

The charter trip to Costa Rica is a barebones one-week bargain, as stated above. However, you can add various one-day excursions at reasonable prices. Limon by combined air and train is $36, Sarchi oxcart factory and plantations $9, the volcano trips $12 each. All include pick'up and return to your hotel via minibus with English speaking guide. Lunch is extra, except on the Limon trip where a box lunch is provided.

It takes the better part of a day from San Jose to the Guanacaste beaches, so one should allow a minimum of three or four days for the trip. Compared to midwinter prices in the Caribbean these are good buyes: roundtrip airfare, $18.50; hotel with all meals, $25 per person per day; approximately $6 for the cab ride from Santa Cruz airport.

Costa Rica had a good economy in 1976. Inflation dropped to 15.5 per cent from 20.5 per cent in 1975. Unemployment remained stable at 45.-5.1 per cent all year. Major exports ' after coffee - are bananas, sugar, cotton, beef and offood processing. There are light industry, assembly plants for small electronics, some book publishing, and documentary and TV film production. A beginning effort is being made in the design and production of women's fashions, and INDICA, a San Jose recording company, is producing and distributing local pop music.