WE WERE HALFWAY through a two-day white-water raft trip along the Urubamba River, deep within Peru's Sacred Valley of the Incas. As dusk settled over our campsite, we could see the next day's first rapid, a churning, twisting maelstrom stretching 400 yards downstream.

If there was any doubt left by then that we were taking a "adventure" vacation considerably different than the usual trip to Peru, one look at that foaming rapid erased it.

Three months earlier, we had decided to fulfill a longstanding dream to visit Peru. We wanted to arrange a trip which would offer something more challenging and exciting than the typical package tour involving Lima, Cuzco, Machu-Picchu and perhaps one or two other stops.

The problem, though, was whether to make the trip on our own or with a group. After considerable research, we signed up for a 17-day trip organized by Overseas Adventure Travel, a Cambridge, Mass., firm that specializes in adventure vacations. OAT emphasizes that its trips are not for everyone. Neither, though, are they only for modern-day Edmund Hillarys.

"To us, adventure means moving at a totally different pace than on a regular vacation," said Judi Wineland, co-owner of OAT. "It's a way to experience a country, to get into local areas where most tourists don't go, or even know about."

For us, group travel made sense -- this time. But it might not be the right choice for evreyone. The key determinant, in the case of Peru, should be time. If you have a lot, you would do well to travel on your own, arranging to hook up with groups once you reach Peru for trips along the Amazon River or down the Urubamba, etc.

If time is short, group travel offers advantages. Peru is an extremely difficult country in which to travel. The roads are poor, and it takes an enormous amount of time to cover relatively modest distances. In addition, since a civilian government took over the country last year, rail and bus workers have been exercising their new right to strike rather randomly and frequently, causing havoc for tourists on a tight schedule.

There is no way we could have accomplished on our own in 17 days what we did with OAT. We never had to worry about details like how to get from place to place, where to stand and where to eat. But group travel is not without its drawbacks. You are generally locked into a schedule offering limited opportunities to free-lance, which can be frustrating if you have a roving nature.

OAT had put together an ambitious itinerary involving seven days of camping, eight days in hotels (usually offering spartan accommodations, i.e., erratic hot water, lumpy mattresses, etc.), and one overnight train. We traveled by bus everywhere, except for the two days on the Urubamba and the 12-hour train ride from Arequipa to Puno.

Our trip involved only a modest amount of hiking, especially as adventure travel trips go. But we did a great amount of driving into remote, hard-to-reach and hard-to-find places, a hallmark of good adventure travel vacations. The cost for the package was $1,600, including the $550 round-trip air fare from New York. Once we were on the road, we spent little additional money, except to buy handcrafts, snacks, etc.

Most of our meals were included in the package. For lunch, we would generally pull off the road and have a picnic of fresh vegatables, bread, local cheese and tasty local fruits. The picnic sites were selected with care. Once we ate just outside the majestic walls of the Temple of Viracocha, a sprawling, though, rarely visited, Inca ruin. Another time we had lunch on a deserted beach.

When we weren't picnicking we ate lunches and dinners at local restaurants where we were usually the only tourists in sight. Along the coast we had some tremendous fresh seafood (corvina) at ridiculously low prices. Occasionally we were left to our devices for dinner, and we would head for the best restaurant we could find. The food, mainly beef and fish, was generally good throughout the country but rarely exceptional.

We steered clear of big cities, for the most part. In fact, we spent only one day in Lima before heading south along the Pan American Highway, one of Peru's few paved roads, through the Chala, or coastal plain region. The four-hour drive itself was unexceptional. But it did give us all a chance to get acquainted. Most of us in our group were personable and outgoing. And the guides were extremely knowledgeable on everything from nature to Inca customs, adding immeasurably to our enjoyment of the trip.

Early on the third morning we hired a local boatman to take us on a four-hour boat ride across the Bay of Paracas, to an offshore island which has become the habitat for tens of thousands of cormorants and boobies and hundreds of sea lions. The island is geologically striking and very beautiful. There are birds and sea lions everywhere and the air is filled with so many squawks, caws and barks that one could be listening to a Charles Ives symphony.

After leaving the so-called "Guano" Island, we took a leisurely drive to Nazca. Situated in a green valley and surrounded by an arid plain, the town is best known as the site of the intriguing Nazca lines.

Scattered for miles across the pampa are geometrically perfect drawings of animals, as well as thousands of seemingly random crisscrossing lines that form a crazy quilt pattern on the desert. The lines and drawings, some of which are 80 meters long, can only be seen from the air. We took a one-hour flight over them the next day. Archeologists differ over their meaning. Some believe the long straight lines make up an astronomical calendar. Others believe the animal drawings were made to appease the pre-Inca gods. Whatever, they are one of the most tantalizing of all archeological mysteries. They must be seen.

Continuing down the coast, we reached the Majes Canyon. Driving off the main road, we wound around a narrow dirt mountain pass, drinking in the astonishing colors and geologic formations in these "mini Grand Canyons" that offer some of the most breathtaking and infrequently seen vistas in the country.

Back on the Pan American Highway, we rumbled into Areequipa, our first city since Lima. Barely five hours later, we boarded the overnight train for Puno, a rather uncomfortable journey in which one must cope with cramped seats, sophistocated thieves and the possibility of developing altitude sickness as the train slowly climbs from 7,000 to 11,500 feet. If you take the train, try to get a cabin. At the very least, go cochet-buffet, which is a step above first class.

Many in our group suffered from nausea and headaches that first day in Puno. By the way, make sure your group's first-aid kit includes oxygen. Also, bring some non-aspirin pain reliever since aspirin and altitude don't mix well. Paragoric, Pepto-Bismol, Lomotil and other anti-diarrhea remedies are also a must. It is virtually impossible to travel in Peru without getting hit by some intestinal problems.

We had been told that Puno and nearby Juliaca -- as opposed to Lima -- were the places to buy alpaca products; so several of us went browsing immediately. In retrospect, we felt that the quality of goods in Lima was higher than in the Indian markets and we regretted not buying more in the capital.

After a day in Puno, we spent one of the most enjoyable and interesting days of the trip on Taquile Island, a three-hour boat ride across Lake Titicaca. Taquile itself is quite stunning, with deep green terraces running up the scraggly, rocky face of the island. Taquile is mentioned in some guidebooks, but few make it seem worth going out of the way to visit. It is.

When we arrived, we hiked to the crest of the island from where we had a fine view of the Bolivian Andes. After catching our breath, we walked to the center of the town. Suddenly, we were taken back in time. For Taquile is a commune and its people, direct descendants of the Incas, run the island in much the same way as Inca villages were run centuries earlier.

It was Sunday afternoon and the islanders were gathered in the main square, a large dirt compound surrounded by small adobe buildings. Leaning against the buildings were the men, clad in heavy white shirts, black pants, beautifully woven cumberbunds and colorful tassled hats. Toward the rear of the square, most of the women sat on the ground, hunched with their dusty black shawls pulled over their heads.

They all listened as the town "mayor," speaking in Quecha, the ancient Inca language, reassigned workers to various jobs, discussed new community business and resolved old problems. When the meeting broke up we wandered around, mingling with the friendly, smiling islanders. Some of us watched with admiration as a few men knit intricately designed hats and vests, while other stopped and played a game similar to Chinese checkers with some young boys.

In the evening we went to the home of a Taquile family. In recent years the islanders have allowed a limited number of visitors to stay overnight in local homes for a nominal charge.

We were directed to a small room, empty except for two crunchy reed mattresses. The family had gone to bed so we did not get a chance to meet them. Instead, we played a game of Scrabble by candlelight before falling asleep. You would do well to bring games like Scrabble and backgammon on an adventure travel trip to pass the time on the occasional long drive.

Over the next two days we remained far off the beaten track, traveling by bus through the Altiplano along an unpaved road so bumpy that one feels as if he has operated a jackhammer for the day. This is the Peru one sees in the pages of National Geographic. We didn't glimpse another tourist the entire drive.

Lush green terraced farmland rolls up to the base of the snow-capped Andes, which are often shrouded in misty clouds. Herds of llama and alpaca, guided by Indian men and women in colorful clothing, line the road. We stopped at a cattle fair in one community and mingled with the local Indians. Later we stopped at a government-run vicuna reserve where we saw these delicate animals from close range.

Finally we reached Cuzco. For the first time in 11 days we stayed in a major city. Cuzco is interesting but paled in comparison to the rugged, colorful countryside we had already seen. However, the Inca ruins outside the city, especially the mind-boggling fort at Sacsahuamna, are worth seeing.

Three days later we left Cuzco to begin our raft trip through the awesome Sacred Valley of the Ncas. The first day was exciting, though not difficult. We camped near Pisac that night, a tremendous Inca ruin whose terraces are perhaps more spectacular than those at Machu Picchu. From the third tier of the fort there is a breathtaking view of the valley below. Unfortunately many people ignore this site.

The first rapid on the second day was graded as aigh 4 or 5 (rapids are graded 1 to 6, with 6 being extremely dangerous for even an experienced crew). It was one mean-looking rapid, even to those of us who had run rapids graded at 5 or 6.

The first raft of seven people navigated the rapid successfully. Ours did not. We immediately flipped, sending seven of us hurtling down the river. The water was so high that we were almost constantly under waves. We confess to wondering as we desperately thrashed about in the Urubamba's brown water whether this adventure travel was really such a good idea after all. But everyone managed to emerge from the river with bodies bruised but intact. Somewhat reluctantly we continued rafting for two more hours.

The following morning we flagged down the tourist train to Machu-Picchu and hopped aboard for the two-hour journey to the famous Lost City of the Incas.

Whatever you have heard or read about Machu Picchu cannot prepare you for actually seeing it. It is one of those rare sights which surpasses one's highest expectations. Situated at 7,000 feet, the city looks down on the Urubamba River. Towering above the ruins is Huayna Picchu, an imposing mountain which also can be climbed in good weather (which we did not have).

We had been on the move for 16 days with hardly a chance to catch our breath. We had seen parts of Peru few tourists experience. We were exhausted. But Machu Picchu made us forget all of that for a few hours.

As we hiked along the Inca Trail for a short distance, we wondered what it would be like to actually speed five or six days camping on the trail, following the path used by the Incas. The prospect was tantalizing. Someday it may become another adventure.

(Besides OAT, several other companies operate adventure travel trips to Peru. They include: Encounter Overland, 369 Pine St., San Francisco, Calif. 94104; Mountain Travel Inc., 1398 Solano Ave., Albany, Calif. 94706; South American Wilderness Adventures, 160 Solano Ave., Berkeley, Calif. 94707; and Forum Travel International, 2437 Durant, No. 208, Berkeley, Calif. 94704.).