My hammock, although it looks the part, is a deathtrap. Woven from synthetic yarn, it is only just wide enough to fit my body, and has already flipped me on to the ground once.

"These hammocks are a bit rubbish."

"Hey, I made that!" says Taylor, a 21-year-old Texan whose thick dreadlocks swing as she turns from playing euchre with friends at the nearby dinner table.

"And that one as well." She points across at Gordon, a middle-aged construction worker from Florida; his leathery body is perched awkwardly on the next hammock along.

At La Iguana - the organic chocolate farm in Costa Rica where I'm staying - many of the fixtures are the products of self-appointed handymen and women. Travellers trade their labour for reduced board as part of the WWOOFing movement (World Wide Opportunities on Organic Farms), working on the cocoa and pineapple plantation, renovating the boarding houses and fixing fences.

On the veranda, Gordon leans over towards me and points at the bamboo roof trusses under which we're lying.

"Bamboo," he says sagely. "You know, you can make almost anything with bamboo if you want to: roofs, walls, floors, supports."

Gordon has come to Costa Rica to set up a sustainable construction business, having packed up his contracting company in Florida following a recent divorce.

"Fourteen years together, married for seven, divorced two and a half months," he says.

Like many of the Americans you meet here, he dreams of buying some cheap, unspoiled land and joining in the green economy. With 12 per cent of Costa Rica's forest designated as national park, and with a peaceful reputation in contrast to nearby Nicaragua and Guatemala, the country has become a playground for US-funded green projects: from environmental courses and workshops to sustainable construction and organic farming. Since February 2006, when the Rainforest Alliance began keeping a database of newly accredited ecotourism ventures for central and South America, 65 of the 166 new entries have come from Costa Rica.

Taylor, idly playing solitaire now that her game of euchre has ended, looks up.

"Who wants to go to the bar?"

We walk in the dark up Mastatal's sole dirt road to la pulpería, the local shop-cum-bar, passing other plantations in this small agricultural village three hours' drive from San José. The group spreads apart, and I find myself talking with Taylor about Rancho Mastatal, the only other place for travellers to stay in the area, run by an American couple, Tim O'Hara and Robin Nunes.

"I wonder if any of the Rancho people will come to the bar tonight," she says as we walk.

"Why wouldn't they?"

"Oh, they have this ridiculous curfew, even though they're adults."

Unlike La Iguana, which is still owned and run by a local farming couple, Lidia and Juan Luis Salazar, Rancho is an architecturally designed ecotourism project that describes itself as an "environmental learning and sustainable living centre". It employs many of the young men in the village on its construction projects, but is detached from the local farming economy, and limits the interaction that its guests have with the local community.

At La Iguana, in contrast, there is a feeling that Lidia and Juan Luis are running their farm much like they always have, with the addition of a dozen or so ecobums. Guests eat with the family and share their kitchen and laundry. The experience is less structured, and there is none of the evangelical atmosphere of Rancho: what one elderly resident at La Iguana witheringly describes as the "Kumbaya spirit".

A year ago, the Englishman Billy Bateman, an ex-financier-turned-globetrotter, helped Lidia and Juan Luis to reconfigure their farm along organic lines to attract WWOOFers. There are still creases to be ironed out, though. When Kat Cline and Doug Christen, organic farmers from Ohio, stayed at La Iguana recently, they discovered that the pineapples were cultivated from cloned stock - a non-sustainable practice. Taylor moans that the compost heap has failed to form decent mulch three times running, due to poor maintenance. Often the farm's green credentials seem to stem more from coincidence than design: mute rusticism and conventional farming methods repackaged as progressive environmental practice. Ecotourism is a draw card for foreigners, and with La Iguana's small crop of cocoa selling for only $1 a bag, and the dozen-odd WWOOFers paying $9 a night for food and accommodation, it is possible that attracting guests here generates more money for the farm than the sale of agricultural produce.

As Taylor and I reach la pulpería, we turn off our torches, which have been scanning the ground for the fer-de-lance, a highly venomous nocturnal snake. We settle down on our stools, and after several drinks the guests from Rancho Mastatal unexpectedly veer out of the darkness, dressed in drag. They have just had a cross- dressing bonfire party, and have been "allowed" out for the night. They join the La Iguana crowd and some local young men (none of the tico women is present), dancing the merengue and clutching their spilling fake breasts and false moustaches while amused farmers look on, sipping Imperial beer.

Afterwards, at a party held at the house of a local father of one, the young men of the village make increasingly bold advances towards female volunteers, until one of the girls is accidentally thrown through a bedroom doorway to the floor - and people eventually decide to leave, our group heading back to La Iguana.

That night, another of the local men, Carlos, is caught trying to get into some of the girls' beds, creating a furore the next morning as Juan Luis and Lidia lecture him on sexual etiquette. He is asked to leave, and is obliged to abandon his plans to stay on the farm in a hut with his brother Graben during his holidays.

As I walk into the kitchen for breakfast, I interrupt Taylor telling Lidia and her son Jorge her version of the story.

"And he was right there! Right next to my bed. He was saying, 'Taylor, Taylor, I'm want having sex with you.'"

Sensing trouble brewing, Lidia is already making chocolate fondue.