Explore America's last frontier with a low-impact excursion into Denali National Park. Cruise through Kenai's fjords on a small ship, followed by a raft trip down the Kenai River, teeming with salmon, in the Arctic National Wildlife Refuge. Alaska Wildland Adventures donates 10 percent of its earnings to wildlife organizations such as the Alaska Conservation Association.
Trip Tip: Opt for the safari-like bus ride (instead of a flight) on the 95-mile road into Denali, where wolverines and moose roam.
Luxury Level: After an evening at Kenai Fjords Wilderness Lodge (kenaifjordslodge.com), on a private island, guests stay at the Kenai Back-Country Lodge, a former 1930's hunting outpost with a lakeside wood-fired sauna.
Alaska Wildland Adventures; 800/334-8730; alaskawildland.com; eight days from $4,295 per person, all- inclusive; June–September.
Kayak the Great Northern Peninsula's shores and hike the 1,000-year-old Viking settlement L'Anse aux Meadows. Along the way, you'll dine on wild caribou and native lobster. Butterfield & Robinson gives a percentage of its profits to local environmental organizations.
Trip Tip: Trek to the abandoned fishing villages of Ireland's Eye and Curley's Harbour, untouched since the '60s.
Luxury Level: You'll spend an evening at the secluded 1865 lighthouse overlooking Iceberg Alley, then three nights at the soon-to-be-wind-powered Fishers' Loft Inn (fishersloft.com).
Butterfield & Robinson; 866/551-9090; butterfield.com; $7,995 per person, all-inclusive; July 7-13 and 14-20.
Travelers on the 11-day trip to Abaco Island track bottlenose dolphins, beaked whales, and West Indian manatees. Your research helps to shape island conservation plans, and 100 percent of the trip fee goes back into Earthwatch-sponsored programs.
Trip Tip: Arrange a tour with Arkwild (242/367-4805; arkwild.org; from $50) to the 3,800-acre pine forest where the last eight Abaco Barbs—descendants of Spanish horses—still live.
Luxury Level: A three-bedroom beach house is shared with seven other people in Sandy Point, Abaco's main town.
Earthwatch; 800/776-0188; earthwatch.org; $2,549 per person, all-inclusive; 11-day trips depart January 8, February 26, and June 10 and 24.
The Sierra Club organizes weeklong eco-immersion trips on the island of St. John. Amid the dense rain forests of the Virgin Islands National Park, volunteers assist archaeologists in restoring trails and 18th-century plantations, and attend lectures on eco-living.
Trip Tip: Bring your snorkeling gear to explore Trunk Bay, which has a 225-yard underwater route marked with plaques identifying the different fish species.
Luxury Level: Although the two-bedroom beach cabins are rustic, volunteers have a stretch of white sand practically to themselves.
Sierra Club; 415/977-5500; sierraclub.org; $1,175 per person, all-inclusive; February 9-16 and 16-23.
Hike the 6,400-foot Bjelašnica Mountain to visit three historic villages—Stanari, Dubocani, and Lukomir—that were temporarily abandoned during the Bosnian war. Locals show travelers how to cook Bosnian meals and weave traditional rugs.
Trip Tip: In Lukomir, trek through the 2,624-foot Rakitnica Canyon.
Luxury Level: You can choose to sleep in locals' houses or camp in tents.
Green Visions; 387/3371-7290; greenvisions.ba; five days from $500 per person; June through September.
Tour the country's capital, Yerevan, and see the Matenadaran archives, one of the world's richest collections of ancient manuscripts. Then travel by minibus to Gyumri, about 60 miles northwest, for language lessons and a stint documenting buildings damaged in the 1988 earthquake.
Trip Tip: Yerevan's Vernissage market (Republic Square) is an ideal place to pick up intricate handmade tablecloths.
Tread softly through Europe by traveling via train. A veteran guide will map out local secrets, from St. Denis's market near Paris to the 377-year-old Eggenberg brewery in the Czech Republic's village of Ceský Krumlov.
Trip Tip: Donate to local charities through the Intrepid Foundation and it will match every dollar.
Luxury Level: Think apartments in Prague and home-stays in Bled, Slovenia.
Intrepid Travel; 866/847-8192; intrepidtravel.com; 34 days from $3,565 per person, all-inclusive except meals; Saturday departures year-round.
Get a dose of colonial culture in Antigua and teach children how to read at schools in the nearby villages of San Andrés Itzapa and Santa María de Jesús. Every morning, Global Vision staff picks up participants and shuttles them to these Mayan towns, where the company has helped more than 450 kids.
Trip Tip: Swing by Frida's (5 Avda. Norte 29; 502-9/782-0504; dinner for two $25), for taquitos and burros.
Luxury Level: Local families host volunteers in homes around Antigua; you're guaranteed a safe and comfortable stay, since Global Vision screens all host families.
Global Vision International; 888/653-6028; gviusa.com; from $2,070 per person, all-inclusive; month-long trips year-round, starting every Saturday.
Spend nine days migrating with native monarch butterflies—the best time to witness their flight is January. Head west to Parque Nacional Eduardo Ruiz in the city of Uruapan ("Eternal Spring") named for its lush vegetation—and venture to surrounding colonial villages, such as Angahuan, on low-impact foot trails. The highlight of the trip is the climb to El Rosario, a thriving butterfly sanctuary where millions of the delicate creatures inhabit a 10,000-foot-high protected forest. Great Adventure People (G.A.P), in partnership with the sanctuary, has planted hundreds of trees so that the butterflies can migrate safely.
Trip Tip: On your free day in Mexico City, ask your hotel to arrange a visit to the ancient pyramids at Teotihuacán, 30 miles northeast, and, en route, the Basilica of Guadalupe.
Luxury Level: Although the hotels are simple, most are family-run and located in colorful colonial houses from the 1800's.
G.A.P Adventures; 800/708-7761; gapadventures.com; $795 per person, meals not included; January 5-14 and 19-28, February 2-11 and 16-25, March 1-10.
Ride the world's largest land mammals at Abu Camp, a 400,000-acre reserve established in 1990 to help once-captive animals return to the wild. Led by Wilderness Safaris, you'll meet the camp's elephant family—the newest addition is Baby Abu.
Trip Tip: Make sure to ask for the wine list with its many African options.
Luxury Level: Guests sleep in antique mahogany beds in raised tents with private balconies.
Wilderness Safaris; 267-11/807-1800; wilderness-safaris.com; three-day trips from $2,300 per person per night, all-inclusive; year-round.
Since 2001, Habitat for Humanity has sent volunteers to three villages in Jordan for two weeks of building houses, with stops at Roman ruins, grand mosques, and age-old souks. Expect six days of hard work rewarded by traditional meals shared with locals.
Trip Tip: Brush up on basic Arabic with ArabicPod (available on iTunes).
Luxury Level: Teams eat and sleep in villagers' houses.
Habitat for Humanity; 229/924-6935; habitatforhumanity.org; 10-day trips from $1,000 per person, all-inclusive; year-round.
Get to know this lush South Pacific archipelago while lending a hand to the Maori people. Volunteers give children reading lessons; clear trails in the bird sanctuary; or assist scientists at the local Whale Research Center. Rarotonga island's crystal-blue waters are ideal for snorkeling, and its jungle-covered volcano is a good hiking spot, as long as you have a seasoned guide.
Trip Tip: Don't miss the Saturday morning Punanganui market, where stands overflow with tropical fruits.
Luxury Level: The KiiKii Motel has simple yet clean rooms, with kitchenettes and a pool outside.
Global Volunteers; 800/487-1074; globalvolunteers.org; two-week trips from $2,295 per person, all- inclusive; departures year-round.
15 of 20Courtesy of Cousteau Fiji Islands Resort/ Chris McLennan
Anyone can hike through the rain forests and dive along the coral reefs of Fiji's Vanua Levua island, but participants in a Seacology expedition can preserve them. The nonprofit helps conserve the island's biodiversity by funding the building of schools and community centers for Fijians, who in exchange, create no-fish marine zones and forest reserves.
Trip Tip: Seacology's weekly diving trips to Namena Island are a must. The miles of reefs make it one of the top diving spots in the world.
Luxury Level: Accommodations are at the Jean-Michel Cousteau Fiji
Islands Resort (fijiresort.com), where 25 private thatched bungalows overlook Savusavu Bay.
Seacology; 510/559-3505; seacology.org; eight-day trips from $2,360 per person, all-inclusive; August 11-18, 2008.
Spend two weeks in a rugged, diverse landscape. Led by a seasoned naturalist, search for the elusive Brown Kiwi on Stewart Island and trek to Westland National Park—a World Heritage Area covered by vast icefields.
Trip Tip: Watch the waves from the Tasman Sea exploding through Punakaiki's Pancake Rocks at high tide.
Luxury Level: Guests stay at the stylish George Hotel (thegeorge.com) in Christchurch and the intimate Old Convent Bed & Breakfast (theoldconvent.co.nz) in Kaikoura.
Natural Habitat Adventures; 800/543-8917; nathab.com; from $6,395, all-inclusive; January 8-21; February 12-25; March 13-26; November 19-December 2; December 24-January 6, 2009.
With its 14-day itinerary to Antarctica, Abercrombie & Kent gives the climate-conscious an opportunity to assist with global-warming research as passengers deliver equipment to scientists at the Palmer Station. Explore the South Shetland Islands along the way, including remote Penguin Island. The only prerequisite: a $500 donation to the Climate Change Challenge, an A&K initiative in partnership with Friends of Conservation.
Trip Tip: Pack your suit for a soak in the thermal waters of Deception Island's Pendulum Cove.
Luxury Level: There are 108 double cabins aboard the 437-foot Explorer II, each with a private bathroom and climate control. (Make sure to request a walk-out balcony.) The vessel also has a 1,000-square-foot health club and spa.
Abercrombie & Kent; 800/554-7016; abercrombiekent.com; $6,190 per person, all-inclusive; December 8-21, 2007.
G.A.P Adventures takes up to 106 passengers on its low-impact M/S Explorer from Edinburgh to the Norwegian coast—stopping at the Orkneys, Shetland Islands, Bear Island, and Svalbard. Spot enough puffins, minke whales, kittiwakes, reindeer, and walruses—and, if you're lucky, an elusive polar bear—to fill your camera's memory card. You'll also benefit from the knowledge of the ship's team of naturalists during shore excursions. G.A.P asks that travelers donate to the trip-long Dollar-A-Day Program to assist communities visited during the cruise; it will also match funds generated from on-board auctions (items include vintage watercolor sea charts) to support polar bear preservation research.
Trip Tip: Take a dive off the main deck before a sauna. It's as invigorating as it is cold.
Luxury Level: The Explorer's modest cabins include some triples and large suites, each with a private bathroom and windows overlooking ice floes and glaciers.
G.A.P Adventures; 800/708-7761; gapadventures.com; $3,195 per person; June 11-25, 2008.
Explore these biologically diverse islands, still a hub for groundbreaking biological research nearly two centuries after Darwin's first visit. Sign up for a Lindblad Expeditions trip and board either the 80-passenger Polaris or the 48-guest Islander. Watch giant tortoises foraging, swim with penguins and sea lions, and tour the Charles Darwin Research Center in Santa Cruz.
Trip Tip: The best time for snorkeling is February through March, when marine turtles emerge.
Luxury Level: Cabins on the Islander and Polaris are relatively spacious, with private bathrooms. Bonuses: yoga classes and a spa.
Lindblad Expeditions; 800/397-3348; expeditions.com; $4,320 per person; operating weekly.
Veteran guides focus on Yellowstone and Grand Teton's natural history on this overland driving trip. While staying in Yellowstone, participants assist in reversing the annual wear-and-tear from the park's 2.9 million visitors by painting historic cabins, mending flood-damaged sidewalks, and sanding and staining park picnic tables. Tauck World Discovery has led clients into U.S. national parks since 1926, and has contributed more than $1 million to the preservation of these treasures.
Trip Tip: At Yellowstone's Firehole River, don't miss the Grand Prismatic Spring's rising blue steam, which the first mountaineers recorded in historic journals dating to the early 1800s.
Luxury Level: Thanks to a sustained relationship with Yellowstone, Tauck guarantees hard-to-come-by, in-park accommodations. From Old Faithful Inn's 1913 East Wing, look out to Upper Geyser Basin, and dine at the preserve's 117-year-old Lake Yellowstone Hotel.
Tauck World Discovery; 800/788-7885; tauck.com; $2590, all-inclusive; May-Sept, 10 days.