Adventure vacations, the kind of holidays that take you hiking, bicycling, trail riding or rafting, have become phenomentally popular in the past decade. And deservedly so, since they combine good exercise, a sense of challenge, interesting companions and a week or more of fun in the scenic outdoors.

The term "adventure," however, can mean many things. To the hardy, it may be a strenuous climb to the top of a distant Himalayan peak, requiring months of training and preparation and an expenditure of thousands of dollars.

Fortunately, there are gentler adventure trips, for the mostly desk bound, that are closer to home (and cheaper), exploring some of the most spectacular scenery in the North American continent.

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Generally, adventure trips are put together by experienced outfitters, who will take you places you might be unwilling to venture on your own. Few travelers have the ability to navigate their own raft through the white-water rapids of the Grand Canyon; but they can have a lot of fun assisting the guide. Similarly, not everyone feels comfortable about plunging into the backwoods without a leader to point the way out again.

Such outings appeal to people who like to try something new, and not just the young singles crowd. Travelers in their sixties and seventies sign up for week-long hikes or bicycle rides through the countryside, and usually they manage to do quite nicely at keeping up -- and may even lead the pack. Most adventure trips are rated by degree of difficulty, and vacationers should not attempt one that is beyond their physical ability.

An adventure trip does not necessarily mean roughing it. You can, in fact, choose an excursion by the degree of comfort provided by the lodging. Of course, you should expect to sleep in a tent on many rafting trips and on hikes into remote portions of the nation's wilderness areas. But other outfitters have put together inn-to-inn hiking and bicycling excursions, so at day's end you always have a roof over your head and a snug bed to dream in.

One concern often voiced by people who have never taken a group tour is, "Will I like the rest of the people in the group?" It is a little daunting to think that you will be spending your vacation with a group of strangers with whom you will be sharing most of your daytime activities and, probably, all of your meals.

Interestingly, many adventure travelers end their holiday citing their companions as one of the biggest pleasures of the experience. Perhaps it's because there is a kind of natural self-selection process. You sign up only if you look forward to meeting new people.

Group trips make especially good vacations for travelers who can't interest a companion in the pleasures of the outdoors. Instead of dragging along a reluctant adventurer, you join a ready-made bunch of like-minded souls.

Among the many North American outdoor adventure vacations offered this year:

*Trail riding in the Colorado Rockies: The horse does the heavy leg and lung work in the uphill climb over several 12,000-foot passes in the spectacular Maroon Bells/Snowmass Wilderness Area just outside the famed ski resort of Aspen. All you have to do is sit tight and take in the views.

"Black forests of spruce dotted with groves of quaking aspens conceal meadows of fantastic color," says the outfitter, offering a choice of a five- or six-day outing. Days are spent on the trail, nights in tents beside a cascading stream.

The five-day trip from Aspen, July 21 to 25, is $475; the six-day, Aug. 16 to 21, is $550. Each includes horse and saddle, riding instruction, tent and all meals, prepared by the cook. Groups are limited to no more than 10 participants, and no prior riding experience is necessary.

For information: American Wilderness Experience, P.O. Box 1486, Boulder, Colo. 80306, (303) 444-2632.

*Inn-to-inn hiking in northern Vermont: The quaint ski village of Stowe is the jumping-off point for a five-day walk through Vermont's mountainous "Northeast Kingdom," perhaps the wildest region of the state. Nights are spent in country inns.

In this part of Vermont, distances are so great between inns, vans are used to transport hikers partway so that the pace of the walk remains comfortable. The groups, limited to 16 participants, cover between five and seven miles a day.

There is time along the way for a swim every day in a stream or lake, including glacially carved Lake Willoughby, where sheer, 1,400-foot cliffs rise from the water.

Departures are July 13, July 27, Sept. 14 and Sept. 21. The price from Stowe is $375 to $395 per person (double occupancy), depending on departure date, and includes lodging, meals, guides and van transport between inns.

For information: Vermont Hiking Holidays, P.O. Box 845, Waitsfield, Vt. 05673, (802) 496-2219.

*Rafting Utah's Canyonlands National Park: The most remote of Utah's magnificent red-rock national parks, Canyonlands is a fantasy world of spires, arches and other unusual rock formations.

A five-day raft trip down the Colorado River will take you 100 miles through the park from Moab in the north to Lake Powell, formed by Glen Canyon Dam, to the south. Along the way, says the outfitter, "You will camp nightly on sandbar beaches, explore ancient Anasazai Indian ruins, take several daily hikes."

The first 40 miles of the trip are relatively tranquil; the white water begins when the Colorado is joined by the Green River to "make a mad dash through the narrow gorge of Cataract Canyon."

Departures aboard oar-powered rafts are May 5, May 12, July 22 and Aug. 23. The price is $525 from Moab, including all meals, equipment and a charter flight over Canyonlands from Lake Powell back to Moab.

For information: Colorado River & Trail Expeditions, P.O. Box 7575, Salt Lake City, Utah 84107-7575, (801) 261-1789.

*Cycling Virginia's Shenandoah Valley: This five-day trip from inn to inn begins and ends in historic Lexington, one of Virginia's loveliest communities, at the southern end of the Shenandoah Valley.

Participants have a choice of routes -- the long way or the short way -- on a loop route north through the George Washington National Forest. The first day covers part of the Blue Ridge Parkway. Day 4 includes a 12-mile descent from Goshen Pass, the road winding alongside rushing Maury River. Back-water pools make attractive swimming holes in the heat of summer.

There are some steep ascents, but softies can climb into one of the vans that accompany cyclers (and carry their luggage on to the next inn) for a ride to the summit.

Departures are May 18, June 29, July 27, Aug. 17 and Oct. 5. The cost is $495 per person (double occupancy), which includes lodging, dinners, breakfasts and a package of maps. Bicycle rental is $79.

For information: Bike Virginia, P.O. Box 203, Williamsburg, Va. 23187-0203, (804) 253-2985.

*Sailing the coast of Maine: A fleet of 13 windjammers, several of them restored schooners from the coastal trade, carry working passengers on six-day trips to the islands and coves of Maine's ruggedly scenic Penobscot Bay.

Actually, you don't have to work if you don't want to, but that is part of the fun. You become, in effect, the crew, who must pull in unison to raise the anchor and hoist the sails each morning. And sooner or later, the first mate will approach looking for someone to swab the decks. They still do that on sailing vessels.

The itinerary depends on the prevailing winds, but often the windjammers call at Maine's fishing villages, and at least once during the week there's a Down East lobster bake at some secluded beach.

Cabins usually are spartan and small, and you may have to do without a shower or private toilet. But the cruises are relaxed and informal, and the two-masted schooners carry only from 20 to 38 passengers. The cost, depending on the ship, ranges from $350 to $465 a week per person (double occupancy), including all meals. Departures are from Camden, Rockport and Rockland every Monday from June through September.

For more information: Maine Windjammer Association, Box 317P, Rockport, Maine 04856, (800) 624-6380.

*By foot and sail through the Hawaiian Islands: This is a 14-day trip into the Hawaii of the past, away from the high-rise resorts and crowded beaches. Visiting four islands -- Kauai, Hawaii, Maui and Lanai -- participants hike some of the state's most scenic trails: through hidden river canyons, along coastal highlands and up the slopes of soaring volcanos. One day, the group, limited to 16 persons, sails in a catamaran between Maui and Lanai.

Throughout the trip, you are never very far from a beach for snorkeling or a freshwater pool for swimming. Lodging is in tents or cabins.

Departures every month, including March 9, April 13, May 18, June 22, July 13, Aug. 3, Sept. 14, Oct. 5, Nov. 9 and Dec. 22. The price from Lihue (on the island of Kauai) is $1,125 per person, including lodging, meals, camping equipment and between-island air fare. The trip ends in Honolulu on the island of Oahu.

For more information: Pacific Quest Inc., P.O. Box 205, Haleiwa, Hawaii 96712, (808) 638-8338.

*Backpacking in New York's Adirondacks: An inexpensive, nine-day adventure, this hiking and camping trip into the Adirondacks of upstate New York is rated as easy-to-moderate.

"Exhilarate in the freshness of mountain forests, swim in clear lakes and wander the fascinating trails," says the organizer, American Youth Hostels. Despite the name, tours are open to adults also.

The departure point is Lake Placid, site of the 1980 winter Olympics. A trip for adults only (age 18 and above) is scheduled for Aug. 9. A trip open to all ages is scheduled for Aug. 16. The cost is $210 per person, including all meals and a tent. Participants must bring a sleeping bag.

For more information: American Youth Hostels, National Administrative Offices, P.O. Box 37613, Washington, D.C. 20013-7613, (202) 783-6161.

These outdoor adventures are only a very small sampling of the trips available this year. To obtain information about other trips, consult:

Specialty magazines that focus on an outdoor activity, such as backpacking, canoeing and sailing. Outside magazine has a particularly good list of trips in its Expedition Services Directory.

Conservation organizations, many of which schedule a program of group tours. Among them are the Sierra Club, 730 Polk St., San Francisco, Calif. 94109, (415) 776-2211; the American Forestry Association, 1319 18th St. NW, Washington, D.C. 20036, (202) 467-5810; and the American Wilderness Alliance, 7600 E. Arapahoe Rd., Suite 114, Englewood, Colo. 80112, (303) 771-0380. defbox For rafting the rivers of the West, the Western River Guides Association, 7600 E. Arapahoe Rd., Suite 114, Englewood, Colo. 80112, (303) 771-0389.

The state tourism office in the capital of the state you are interested in visiting. Ask for a list of outfitters who lead the kind of trip you want.