Seeing it out of the corner of your eye when you weren't really paying attention, your first thought was: It's a flying saucer. As such, it blended perfectly into the San Francisco cityscape.

Actually, it was something almost as unlikely: the first regularly scheduled, fare-paying passenger dirigible in America since the Hindenburg exploded in 1937. From last May until two weeks ago, Airship Industries' 12-seat blimp would depart Oakland International Airport, amble over the city of Oakland, follow the Bay Bridge to Treasure Island, skirt around San Francisco and head up past the Golden Gate Bridge. Once there, it would turn slowly around and then follow roughly the same route back, dipping down low over Alcatraz.

It was a terrific trip. Except during the ascent, passengers were free to walk around the cabin and even lean out the windows. ("Take care not to drop glasses or cameras into San Francisco Bay," the pilot warned. "That throws off everyone's yachting holiday.") The ride wasn't particularly quiet, but it was more gentle than any jaunt on the Eastern Shuttle.

And while a blimp is undoubtedly the most appealing way to see a city from on high, it is just one of many methods of touring by air -- a travel trend that, despite some controversy, has taken off in the last few years. Helicopters and touring planes offer an opportunity for the elderly, the infirm or the time-pressed to see otherwise inaccessible spots. Ballooning, meanwhile, has been growing in popularity in scenic areas ranging from the Adirondacks to California's Napa Valley to Australia.

San Francisco is widely acknowledged to have the best setting of any city in North America, and the blimp trip -- which generally took an hour, give or take a couple minutes -- showed it off to best advantage. As the craft left its mooring station, the city floated off in the distance, partially hidden in the mist. The closer the approach, however, the more the office buildings, hotels, houses and hills were revealed. At this elevation (about 800 feet, much less than the height of the Empire State Building) no binoculars were needed to reveal the details.

The price for the ride was $150. No champagne was provided on board -- just cans of Slice and other Pepsico drinks -- but otherwise it was a fair deal. Unfortunately, it is a deal that was abruptly canceled. "When we were presented with the declining weather in San Francisco this time of year coincidental with the request of a large sponsor, Pepsi, to do a tour of the country, we decided to move the blimp," says George Spyrou, general manager-international for the blimp's owner, Airship Industries.

While this was a popular ride, only a portion of the money needed to keep the blimp aloft came from paying passengers. Most of the income was derived from the advertising on the side of the dirigible, and there were too many gaps in the contracts. The Pepsico offer, which will last for a year, was just too good to pass up.

Airship Industries, a British company that has offices in New York City, has 11 blimps around the world. At the moment only one, in Sydney, Australia, is carrying paying passengers. However, this spring and summer, there's the possibility of passenger rides in Munich and London -- cities where the company has successfully operated before -- and in New York, which would be a new market. There's also the possibility of an eventual return to San Francisco.

Being up in the air doesn't come cheap. The cost of an hour's tour -- whether blimp, helicopter, plane or balloon -- usually ranges between $100 and $150 per person. And while no one seems to protest too much about balloons, helicopters and planes are a different matter.

Much of the controversy has focused on the Grand Canyon, where dozens of companies offer more than 50,000 flights a year. In 1984, 10 people were killed when a sightseeing plane crashed in the national park. In 1986, a helicopter and a plane collided, killing 25 people.

In response to those tragedies and increasing complaints about noise pollution, President Reagan signed legislation last August banning all flights below the rim of the canyon. Last month, the National Park Service went further, recommending that tourist flights be eliminated over 44 percent of the Grand Canyon.

According to Assistant Secretary of the Interior William Horn, the plan was "the result of our careful evaluation of the competing needs of those who seek air tour services and those who prefer wilderness solitude in the inner reaches of the canyon." The Federal Aviation Administration has until March 9 to adopt these regulations, unless for safety reasons it decides a modification is necessary.

In Alaska and Hawaii, the site of increasing numbers of sightseeing flights, similar restrictive action is being taken. Stephen Ambrose, a district ranger with the Tongass National Forest, says there were several applications in the fall of '86 for helicopter tours of the Juneau ice field, which is part of the national forest.

As a result, he says, "there were concerns about the impact on wildlife, particularly goat habitats, and also concerns about noise levels from home owners who would be under the flight paths."

The solution was to zone the ice field, arranging for the helicopter landings to be widely dispersed rather than all in one section. In addition, some areas were restricted completely from helicopter use.

This won't be the last time the issue arises. "Alaska is so massive, people are looking at ways to access areas that are otherwise remote," says Ambrose. "It stands to reason that the use of helicopters would increase. There's certainly going to be more effort to keep a balance."

In Hawaii, an average of 40 flights a day were crossing Haleakala National Park, often close to ground level. "Hikers, backpackers, campers and horseback riders were complaining about being buzzed by low-flying helicopters, and others complained simply about the noise," says Pete Sanchez, the park's acting superintendent. "People had come thousands of miles to get solitude in the wilderness, and this solitude was shattered."

The legislation banning below-the-rim flights in the Grand Canyon also ordered that all flights over Haleakala be at least 9,500 feet above sea level. Effectively, that means the helicopters must now be at least 2,000 to 3,000 feet above the ground. "It seems to be working," says Sanchez. "There are very few complaints."

A third park directly affected by the law is California's Yosemite. A minimum altitude of 2,000 feet is now required there. In addition, flights over more than 25 other parks will be evaluated in terms of noise and safety intrusions.

Even the San Francisco blimp didn't escape a few minor protests. After a couple Oakland residents voiced their concerns about noise, the flight path of the dirigible was varied to put it over the bay more of the time. And several health-conscious Californians were unhappy when Pall Mall cigarettes briefly used the blimp for advertising.

When you do go on a sightseeing flight in Alaska or Hawaii, what do you get for your money? Two representative, well-established companies are Temsco and Maui Helicopters.

Temsco, headquartered in Juneau, has been flying in Southeast Alaska for 30 years. The most popular of its eight regular helicopter flights is a 45-minute trip to the nearby Mendenhall Glacier, part of the Juneau ice field. Passengers are flown over the 12-mile Mendenhall, where they are set down for a quick bit of exploration. Bob Engelbrecht, Temsco's director of tour operations, describes the experience as "totally different. The ice is a very deep blue, and it's moving -- about two feet a day. You see a 1,400-foot icefall, the high peaks all around you, the crevasses in the glacier itself and occasionally a mountain goat or two. To see this by hiking would take at least a half day for an experienced climber." Cost for the tour, which is available as a shore excursion for cruise-ship passengers, is $105.

Another Temsco trip involves a helicopter ferrying a guide and a group of four to 10 people to a remote ridge or valley, where they hike for several hours or a full day. (The price for this varies.) Other trips leave from Skagway and Ketchikan, visiting glaciers and the Misty Fjords National Monument and tracing the gold rush trail. Costs range between $79 and $159 for 30 to 75 minutes. For more information: Temsco, 1650 Maplesden Way, Juneau, Alaska 99801, (907) 789-9501.

On the Hawaiian island of Maui, the most popular tour offered by Maui Helicopters is its Panoramic Journey. "What we're doing," says marketing manager Ray Serrano, "is showing people Hawaii the way they believe it is. In actuality, it doesn't look like that anymore -- except here. We don't fly around and show people condominiums. We get in the remote areas."

The $135 trip, which takes an hour, leaves Kahului Airport, flies east over the rain forest, passes the old Hawaiian village of Hana and circles back over Haleakala Crater in the national park. The helicopter seats four passengers.

A principal selling point, Serrano says, is that the trip condenses the scenic tour of Maui. "The average traveler does not come here for that long. They come for the suntan, the beach," he says. "Hana is beautiful, but it's a good three- or four-hour trip out there. All told, we can take away a couple of days driving time."

The company does other tours, including a longer, more expensive trip that circles the whole island. For more information: Maui Helicopters, P.O. Box 1002, Kihei, Hawaii 96753, (800) 367-8003.

An enjoyable and safe balloon ride depends entirely on the skill of the pilot. If unpredicted weather or an emergency arises, the balloonist must know what to do. Here are four established companies with professional pilots serving different parts of the world:

The McLean-based Bombard Society says it owns the largest fleet of balloons in the world: 34 of them. This year, tours are scheduled during the spring, summer and fall in both France's Loire Valley and in Burgundy; in Salzburg, Austria; in Tuscany, Italy; Bath, England; and Bern, Switzerland. The idea here is ballooning as an integral part of a luxurious grand tour. Their Loire trip, for instance, is a seven-day affair that includes six ascensions in different areas of the valley. In the morning you visit the cha~teaus; in the afternoon, you fly above them. The all-inclusive cost is $4,585 per person; a "half adventure," which includes three flights, is $2,395. For more information: The Bombard Society, 6727 Curran St., McLean, Va. 22101, (703) 448-9407.

Adirondack Balloon Flights in Glen Falls, N.Y., has a smaller fleet than Bombard: one balloon. Starting in late April or early May, they will be offering rides for the fifth year. Their launch area, about an hour north of Albany, lies in the bucolic valley between Vermont's Green Mountains and New York's Adirondacks. Launches are either at sunrise or a few hours before sunset; a champagne picnic is served after landing. Cost is $135 per person. For more information: Adirondack Balloon Flights, P.O. Box 65, Glen Falls, N.Y. 12801, (518) 793-6342.

Just to the north of San Francisco, Balloon Aviation of Napa Valley typically launches eight balloons a morning from April through November. During the rest of the year, they fly about four days out of seven -- the ones with passable weather. The balloons follow the wind 500 to 1,000 feet above the valley, drifting over the vineyards and wineries. Afterward, there's champagne. Price is $145 per person. For more information: Balloon Aviation of Napa Valley, 2299 Third St., P.O. Box 3298, Napa, Calif. 94558, (707) 252-7067.

To go ballooning in Australia requires a bit of preparatory legwork. So/Pac, a travel wholesaler representing 400 operations in the South Pacific, can arrange for flights in three areas: Brisbane, Sydney and Alice Springs. Flights cost about $100 (U.S.). The Alice Springs flights go over the MacDonnell Ranges in the outback, and can be booked as part of a five-day safari. During the flight, the balloon company says, there will be sightings of kangaroos, wild horses, camels and varied bird life. For more information: So/Pac, 1448 15th St., Suite 105, Santa Monica, Calif. 80404, (800) 551-2012.

Airship Industries may be temporarily out of the blimp-ride business in North America, but it doesn't intend to stay that way. Instead of having to depend on advertisers to support its passenger flights, the company plans to build bigger blimps. Carrying 40 passengers instead of 15 would enable it to reduce the price per person to $100, and would also eliminate the need for any advertising.

The firm has even bigger ambitions. "Within five years, we hope to have a dirigible that can carry in excess of 100 passengers," says general manager Spyrou. "We will do Orient Express-type cruises. Since the key issue with blimps is weight, we wouldn't have people sleeping overnight. But we'd stop at cities, stay in fancy hotels, and the next day fly off again."

One route the company is thinking about is London-Paris-Nice-Venice-Athens-Cairo. To make it work, Spyrou says, Airship is thinking of charging somewhere in the range of $800 a day. That would be all-inclusive, he notes: "We won't nickel-and-dime them."