There's such a wide variety of adventure programs available these days that travelers hardly know where to look.
And since many of the best, offering more personal and customized service, are small operations that have virtually no marketing budgets, they can go unhailed except by word of mouth -- or through lucky mention in one or more of the increasing number of books being published about adventure and nature-oriented travel.
Possibly the best general printed source for adventure-travel information is "The Ultimate Adventure Sourcebook," by Paul McMenamin ($29.95, Turner Publishing).
The 432-page book is divided into 30 different adventure categories. Under each, the author rates programs for service, value and itineraries.
Here are some other general source books and periodicals:
* "Eco Journeys: The World Guide to Ecologically Aware Travel and Adventure," by Stephen Foehr, Noble Press, $14.95.
* "The Buzzworm Magazine Guide to Ecotravel," Buzzworm Books, Boulder, Colo., $9.95.
* "Going Off the Beaten Path: An Untraditional Travel Guide to the U.S.," by Mary Dymond Davis, Noble Press, $15.95.
* "The Green Travel Sourcebook: A Guide for the Physically Active, the Intellectually Curious, or the Socially Aware," by Daniel Grotta and Sally Wiener Grotta, John Wiley Inc., $14.95.
* "Earthtrips: A Guide to Nature Travel on a Fragile Planet," by Dwight Holing, Living Planet Press (Conservation International), $12.95.
* "Adventure Travel North America," by Pat Dickerman, Adventure Guides Inc. (7550 E. McDonald Drive, Scottsdale, Ariz. 85250;  252-7899), $20.
* "Eco Vacations: Enjoy Yourself and Save the Earth," by Evelyn Kaye, Blue Penguin Publications, $22.50.
* "AdvenTOURing: A Source Book for Adventure & Special Interest Travelers," by Shirley Deeter, R&E; Publishers (P.O. Box 2008, Saratoga, Calif. 95070;  866-6303), $8.95.
* "Environmental Vacations: Volunteer Projects to Save the Planet," by Stephanie Ocko, John Muir Publications, $16.95.
* "Volunteer! The Comprehensive Guide to Voluntary Service in the U.S. and Abroad," Council on International Educational Exchange, $8.95.
One periodical to look for is Specialty Travel Index, semiannual, $10 per year (in travel bookstores or newsstands, or call  459-4900). It's a well-indexed listing of adventure-tour operators.
Just Go! is a new quarterly that concentrates on environmentally and culturally sensitive tourism, usually offering tips for such options as home stays and involvement in ecology projects. Annual subscription is $12.50; find it on newsstands or call (800) 285-5951.
Other titles to look for on newsstands include: Buzzworm, the Environmental Journal; Outside; Transitions Abroad; Backpacker; Audubon; Sierra; Great Expeditions -- not to mention specialty publications for activities ranging from diving to climbing.
Once you've decided where you want to go and how you want to travel, there's probably a specific guidebook to help.
For the Pacific Northwest, for example, and, increasingly, for worldwide destinations, various books and series by the Seattle-based Mountaineers Books will cover most muscle-powered, outdoor activities.
New titles or editions include: "Costa Rica's National Parks and Preserves: A Visitor's Guide," by Joseph Franke, $16.95.