Once upon a time, a vacation was designed to provide a site for non-stop sloth. Whether it was spent beside a sandy beach or a mountain lake, the only energetic activity that took place during the holiday was the rhythmic scratching of one's stomach. But the world has changed. Vacation times have become increasingly more active. It's not uncommon now for a returning vacationer to need a few weeks off to recover from the rigors of his holiday. Holiday packages have begun to vie with one another for the degree of risk and danger they offer. You can choose your special poison from an assortment of potential body breakers that would scare the ace bandage off an orthopedist. Intrepid adventurers can risk their well-being by soaring in the sky, hanging from a mountain cliff, or being submerged under hundreds of fathoms of ocean. There's an endless array from which to choose; what follows is merely a sample.

Cube Climbing

One of the easiest ways to avoid suffering heat prostration from summer adventure excursions is to concentrate your activities amid the snow- and ice-covered peaks of the Cascade Mountains. It would be gratuitous to point out the proximity of these summits to Mount St. Helens, so let's concentrate instead on the six-day snow and ice climbing instruction that hones the skills necessary for difficult alpine ascents. Participants are instructed in a climbing technique that includes the best of French, German, and American approaches to mountaineering in snow and ice. The level of technical acuity is high, and according to the description "the curriculum includes glissade skiing, use of ice axes, crampon techniques, ice hammers, and direct-aid climbing with jumars, ascenders, and prussiks." Sounds good to me. During the summer, climbers sleep in tents; in winter, accommodations are in snow caves and igloos. No need to worry about excessive pampering.

Cost: $250-$275 includes instruction, guide service, technical equipment. Dates: May to December. Information: North Cascades Alpine School, 1212 24th I, Bellingham, WA 98225; (206) 671-1505.

Jet Wake

The introductory brochure is hardly confidence-inspiring: "Jet boating is a noisy way to cruise. It consumes fuel. And on rare occasions an underwater rock will rip a hole in a boat's bottom and it will sink." Then it lists all the jet boating tours. A typical trip includes a run through the waters of the granite-walled Salmon River Canyon in Idaho, a geological phenomenon a full fifth of a mile deeper than the Grand Canyon. There are day trips and overnight excursions to the China Bar Lodge. And there's sufficient time to allow passengers to pan for gold, explore old abandoned homesteads, and search for Indian artifacts. Swimming and fishing are other diversions, though the real excitement is provided by the swift navigation of the river itself. The speed and the ceaseless thunder of the river in its wild, white-water patches make for some heart-stopping incidents. It's a lot different from floating along in an old inner tube.

Cost: Day trips start at $50; overnight excursions start at $70 all-inclusive. Dates: February through November. Information: Bob Smith's Salmon River Boat Tours, Box 1185, North Fork, ID 83466; (208) 865-2512.

High Times

It's not unusual for adventure travelers to want to get up in the world, and this vacation may be the ultimate high. Hang gliding in the Rockies can mean that your jumping-off point is as high as 14,000 feet above sea level, and the thermals can take you straight up from there. Would-be soarers, from beginners to experts, are accommodated just outside of Denver, with lessons, including group soaring clinics, available year-round. The courses include a basic five-lesson package, a towing course, an advanced mountain session, and a high-performance course for Hang III and IV pilots. The surrounding Rocky Mountain terrain provides nearly unlimited opportunities to soar, among the snow-capped peaks or down through the deep mountain valleys.

Cost: $80 for the high-performance course to $200 for the basic package. Dates: Year-round. Information: Golden Sky Sails, 572 Orchard St., Golden, CO 80401; (303) 278-9566.

Wink At The Extinct

How often do you get the opportunity to have lunch with a dinosaur? This social coup can be yours if you're willing to float down the Green and Yampa rivers through Dinosaur National Monument. This area contains a great concentration of fossils and other petrified remains, marking the era when these species last roamed the central United States. Trips begin and end in Vernal, Utah, and itineraries are flexible. Such tame pursuits as taking photographs and swimming in the swift, cool rivers are combined with substantial rock climbing and hiking to remoter areas to see the best of the ancient relics.

Cost: From $29 (one day) to $270 (five-day trip), includes food, equipment, guides. Dates: June to September. Information: Hatch River Expeditions, Box C, Vernal, UT 84078; (801) 789-4316.

Vroom, Vroom

This is a new sort of saddling-up opportunity in the Southwest. Instead of using pinto ponies, these newfangled cowboys ride BMW R65 or R80/7 motorcycles or Ford Broncos. A particular favorite with European visitors to America, this is the high-speed way to see a good portion of the old Wild West, including cowboy-and-Indian country, from the southern Rockies to the Mexican border. Luggage is schlepped in a pickup vehicle so riders can explore terrain too rugged for more conventional means of transportation. Precise routes vary according to the time of year and the skill of the motorcycle riders in the group. Although such sites as the Grand Canyon are a staple of tour itineraries, most other sites are very much out of the ordinary. Participants can choose either a one- or two-week segment of the bike or bronco-back trips, between February and the end of November. The only element consistent with the old horseback treks through this part of the country is the tradition of saddle sores.

Cost: One-week vacations range from $490 to $790; two-week vacations range from $990 to $1,590. Dates: February through November. Information: Western Adventures Inc., 6141 Camino Almonte, Tucson, AZ 85718; (602) 299-4691.