(Business 2.0) – Count Dolphins EARTHWATCH INSTITUTE www.earthwatch.org; 800-776-0188

What You'll Do: Spend four to eight hours every other day on a boat identifying and counting dolphins and whales off Great Abaco Island in the Bahamas. Back onshore, you'll develop film of the animals or enter data into a computer.

How It Helps: Scientists think sonar is disturbing these sea mammals. An accurate map of their movements might help to prove the theory.

Dates: June 3-13, June 17-27, July 1-11, July 15-25, and July 29-Aug. 8. The nonprofit group also offers 135 other environmental and archaeological expeditions in 48 countries. Projects with "charismatic megavertebrates"--lions, elephants, and gorillas--are the most popular.

Cost: $1,845, not including transportation.

Field Conditions: You'll live in a dormitory-style beach house equipped with showers and toilets. You and fellow volunteers prepare your own meals of local fish, conch, and fruit.

Tip: Though tropical, the Bahamas are in the Atlantic, not the placid Caribbean. If you're susceptible to motion sickness, don't forget the Dramamine.

Hack Out Hiking Trails AMERICAN HIKING SOCIETY www.americanhiking.org; 301-565-6704

What You'll Do: Rake, shovel, and chop six to eight hours a day in the stunning Tonto National Forest northeast of Phoenix, cutting trails near prehistoric cliff dwellings.

How It Helps: A well-built trail prevents erosion and opens up public land.

Dates: June 22-28. The group also coordinates more than 80 other projects in national parks and forests, including challenging Alaskan mountain ranges and less strenuous ones in Virginia's Mt. Rogers National Recreation Area.

Cost: $80 for society members, $100 for nonmembers. You also have to pay your own way to Phoenix.

Field Conditions: The accommodations are strictly BYO, in the form of a tent and sleeping bag, though all meals are provided.

Tip: You must be able to hike 5 miles or more into the site, so train by walking that far at least two or three times a week.

Build Homes for the Maori HABITAT FOR HUMANITY INTERNATIONAL'S GLOBAL VILLAGE www.habitat.org; 229-929-6935

What You'll Do: Pound nails for eight hours a day, building a frame house for a Maori family in the little town of Rotorua on New Zealand's scenic North Island. The region is full of geysers, thermal springs, and boiling mud pots, and yes, you'll be given time off to explore them.

How It Helps: Though New Zealand is a relatively wealthy country, the indigenous Maori face poverty and substandard housing conditions; many are living without basics such as running water and electricity.

Dates: March 28-April 13. If you can't make this trip, join Habitat in building one of several more homes in New Zealand or 20,000 others around the world this year.

Cost: $1,600, plus airfare to Auckland.

Field Conditions: On Habitat's projects in less developed countries, it's strictly squat toilets and bucket showers. But the plumbing's no problem here: You'll stay in a hotel and in the home of a local Kiwi family.

Tip: If you want to work alongside Jimmy Carter, join the massive Georgia and Alabama Habitat project June 6-13.

Preserve an Old Village LA SABRANENQUE RESTORATION PROJECTS www.sabranenque.com; 716-836-8698

What You'll Do: Restore the medieval French village of St. Victor la Coste--which involves carrying stones and building walls. Afternoons, though, are for seeing sights such as the Roman aqueduct Pont du Gard and wandering the picturesque villages of Provence.

How It Helps: You'll preserve a key collection of rural architecture from the Middle Ages, while participating in a hands-on study of thousand-year-old construction and carpentry techniques.

Dates: Two-week sessions from June to September. Five-day sessions from March to May and again in October.

Cost: $580 for two-week sessions, including housing, meals, and jaunts into the countryside. Five-day sessions cost $430.

Field Conditions: You'll sleep two to a room in restored houses decorated with French Country furniture and eat hearty Provencal meals prepared by a local chef.

Tip: English is spoken as much as French, so if your goal is to polish your linguistic skills, this may not be the holiday for you.

Track Sea Turtles CARIBBEAN CONSERVATION CORP. www.cccturtle.org; 800-678-7853

What You'll Do: Travel to remote Tortuguero on the northeast coast of Costa Rica's rainforest to count the nests and eggs of the endangered leatherback turtle and the rarer green turtle. You'll hike as far as 6 miles daily, but it will be by moonlight on a remote black-sand beach.

How It Helps: The biometric data you'll gather aids scientists in their understanding of the turtles' life cycles. Your presence on the beach also helps to deter poachers.

Dates: One-to three-week programs from March to June for leatherbacks and June through November for green turtles.

Cost: $1,360-$2,180, not including airfare to Costa Rica.

Field Conditions: Dormitory-like digs have hot running water and screens to keep the insects at bay.

Tip: September is the time to see green turtle hatchlings, April for the leatherbacks.

Entertain Sick Children HOLE IN THE WALL GANG CAMP www.holeinthewallgang.org; 860-429-3444

What You'll Do: Teach lanyard braiding and campfire songs to kids suffering from cancer, sickle-cell anemia, hemophilia, HIV, and other serious diseases.

How It Helps: The 300-acre camp, founded by Paul Newman, has a medical director, five nurses, two doctors, 30 counselors, and more than 40 additional program staff members, but it still needs help with the 120 kids who attend each session.

Dates: Nine sessions, from June through August, last between seven and nine days.

Cost: Just your transportation to and from the camp, located in Ashford, Conn.

Field Conditions: Eight campers, two staffers, and two volunteers share a log cabin with full plumbing. Each cabin has a phone and a golf cart in case of emergency.

Tip: The camp accepts some 300 volunteers, but far more women than men sign up. If you're male, you might still get in this year; if not, now's the time to apply for 2004. --LOIS GILMAN