On Thanksgiving weekend, the U.S. Navy plans to host a special Halley's comet watch at Windmill Beach. For a small donation, participants can gaze through binoculars at the famous celestial streak, quaff beer or buy a T-shirt. The proceeds are earmarked for the local PTA.

It will be a typical American outing, except for one conspicuous difference: The road to Windmill is flanked by antitank ditches and mine fields.

The Navy base on this southern coast of Cuba is a curious blend of small-town U.S.A. and armed camp -- suburbia on a fault line of the Cold War. It is an anomalous place where the 18-hole golf course is surrounded by 18 miles of barbed-wire fence, the yacht club shares an azure Caribbean Sea with communist gunboats, and Baskin Robbins' 31 flavors store is defended by Stinger antiaircraft missiles.

Twenty-six years after Fidel Castro's revolution transformed Guantanamo into a geopolitical lightning rod, the Navy warily occupies the only U.S. military facility on communist soil -- 45 acres of strategically prime land leased in perpetuity to the United States under President Franklin D. Roosevelt's Good Neighbor Policy.

"Gitmo," as the Navy calls the base, fell on hard times in the late 1970s as the Carter administration cut its budget and manpower and questioned the usefulness of a base that Castro and his Soviet benefactors have called a symbol of "Yankee imperialism."

But President Reagan, determined to bolster U.S. power in the troubled Caribbean, has renewed the commitment to Guantanamo.

Last year's $44.3 million budget for base operations nearly doubled the spending limit of five years earlier. The Navy plans to spend $40 million for new housing, commissaries and other facilities to improve the life style on the hilly, arid compound for its 6,000 servicemen, dependents and civilian employes.

On a media tour of Guantanamo organized by the Navy last week, the guide proudly pointed out steel girders where the first McDonald's is being erected.

"Guantanamo is a highly visible reminder of our resolve in the Caribbean," Capt. John R. Condon, the base commander, said.

"Would the Soviets be here if we weren't?" he asked. "All you have to do is read the history books. Cam Ranh Bay. Da Nang," he said, citing two Vietnamese ports opened to Soviet ships after the communist North seized power throughout Vietnam.

As a strategic asset, Guantanamo points like a finger into the Caribbean, whose sealanes carry two-thirds of U.S. oil imports and other important raw materials. It affords the Navy an eye on Soviet ships steaming to Cuban ports.

The largest U.S. naval fleet training base in the world and a supply center for the Atlantic Fleet, Guantanamo is growing in importance as the Navy nears its 600-ship goal.

Even as the Navy digs in, Guantanamo is enjoying unusual tranquility. The very name once stood alongside Berlin and Quemoy, the Nationalist-held island off mainland China, as code words for superpower tension.

Gitmo was an international flashpoint throughout three decades. In 1959, its gates were closed; in 1962, the Cuban missile crisis forced evacuation of American civilians, and in 1979, 1,800 Marines staged a military exercise in response to the presence of Soviet troops in Cuba.

"Maybe we're finally being accepted as neighbors," Master Chief Tony Summerlin said. "There's no tension between us and the Cubans these days."

The Navy tries hard to foster the tranquility. Sentries man high observation towers with M16 rifles but require a superior's permission to load their weapons. Base officers avoid embarrassing Castro by refusing to divulge the number of Cuban defectors who jump the fences into Guantanamo.

Castro refuses to cash the $4,085 monthly rent check tendered by the United States, but Condon noted pointedly that "the checks are not voided. They're still good."

The picture of normalization depicted by the Navy is marred by numerous signs of danger. Planted along the pink oleanders and squat sea-grape trees dotting the roadsides are more than 50,000 land mines marked by warnings in Spanish and English.

Throughout the base, an invading enemy would face piles of scrap metal designed to stop tanks in what the Navy calls "anchor valleys." Rows of underground tubes called "blow holes" flank the narrow highway that winds through Guantanamo, ready to be filled with explosives.

Nothing better symbolizes the uneasy peace here than North Gate, once the main thoroughfare for thousands of Cubans working on the base and for U.S. servicemen who frequented the bordellos and bars of neighboring towns.

North Gate was closed in June 1959, after Castro took power in Havana, and is guarded today by the main concentration of 400 Marines in camouflage fatigues. They defend the base perimeter against about 1,200 Cuban troops stationed in the scrubby frontier surrounded by the steep Sierra Maestra mountains.

Col. Sam Adams, a tough-talking Texan who commands Marine Ground Defense Forces here, said that, except for occasional rocks thrown on the barracks roof by "Cuban rednecks," his post remains calm.

"The chances of Castro ever coming across are remote," Adams said. "If they do, we're prepared for them. Like they say in Texas, 'One riot, one ranger.' "

To deter defections, Cuban authorities have placed four barbed-wire fences and rows of cactus -- called the "Castro barrier" -- outside Guantanamo. Still, Adams said, at night "you see searchlights, you hear shots, you hear screams."

About 60 Cubans known as "commuters" pass daily through North Gate, the only human bridge between the two worlds. Hired before Castro seized power, Cuba allows them to work on the base as long as they return home at night and convert 90 percent of their earned dollars into pesos.

Rasman Henry Cook, 61, who lives in a small town outside Guantanamo, retired recently after 42 years as a pipefitter. He earned $300 a week, many times the average Cuban wage.

As he left North Gate for the last time, Cook said the political barriers here have been accepted as "a way of life." Recalling days when Guantanamo was inseparable from the rest of Cuba, he said, "Those gates have been closed a long time.