Pleasure Island

""You boys have had your fun -- NOW PAY FOR IT!""

- The Coachman, Pinocchio

This would be an event or activity that is presented to one or all of the protagonists as being extremely enjoyable. This event or activity never has any redeeming social value; it's just for fun. It's usually presented as being completely innocuous and not at all objectionable - except, of course, in those cases where the basic selling point of the "fun" thing is that it's something the "unhip, uncool" people of the world would never allow one to do. After perhaps some initial reluctance, the protagonists decide to give it a shot. Bonus points are awarded if some "nerdy" character suggests that it's not a good idea, and everyone either ignores or laughs at this person.

When the protagonists finally get involved in the alleged fun, it turns out to be either entertaining beyond their wildest dreams or... well, rather boring. In any case, it certainly doesn't seem to be harmful. But that all changes as the story progresses. Maybe the idea of "fun" being offered up goes way over the line, ultimately breaching the frontiers of good taste or sanity. Maybe the entertainment is actually being used as a cover for some nefarious or destructive purpose. Or maybe the event or activity involves a level of near-suicidal danger that no one could have ever seen coming.

The Trope Namer, of course, is the amusement park in Pinocchio that causes boys to turn into donkeys (which, chillingly enough, shares its name with a "fun zone" at Walt Disney World).

Often overlaps with The Game Plays You, A Fete Worse Than Death, and/or Crap Saccharine World. Broader than Amusement Park of Doom, because it can involve just about anything that is ostensibly entertaining.

Film

 * The 1997 thriller The Game. For his birthday, Nicholas Van Orton (Michael Douglas) gets a unique gift from his brother Conrad (Sean Penn). It's a sign-up sheet for Consumer Recreation Services (CRS), which offers customers the opportunity to play an interactive "game." Unfortunately for Nicholas, the only "playing" that ensues is when a conspiracy of complete strangers spend the whole movie mercilessly playing with his head.
 * In the 2001 low-budget film Zebra Lounge, a happily married couple become bored with their relationship and decide to meet up with a pair of "swingers" in the city for a night of choreographed adultery. It is all perfectly innocuous (if naughty) fun....until the swingers start stalking them.
 * In the 1990 Teenage Mutant Ninja Turtles movie, The Shredder has created a "pleasure island" to lure misguided teens into his Foot Clan. His warehouse is located on the ficticious Lairdman Island and features rap music, pinball/video games, basketball, dancing, billiards, skateboarding, gambling, graffitiing, and tobacco use. Newcomers to the warehouse are told "Anything you guys want... We got. Anything you wanna' do... Do it." The naive protagonist in this instance is Danny Pennington (Michael Turney).
 * In Hostel the three protagonists have the chance to spend their holidays in the titular hostel in Eastern Slovakia where they can get any girl they want. As soon as they get there, they start having the time of their live...until they find out that the hostel iso owned by a sadist crime organisation who kidnaps tourist and offer'em to rich paying customers who pay for rape and/or torture 'em in the most gruesome way.

Literature

 * The Goosebumps book Let's Get Invisible! features a group of kids who find an old mirror that can turn you invisible. Only problem is, if you stay invisible too long, you get sucked into the mirror, and your reflection replaces you in the real world.

Mythology

 * Older Than Feudalism: The Island of the Lotophagi from The Odyssey.

Theatre

 * Rhododendron in The Golden Apple gives this trope even more play than in The Odyssey, which it is a homage to. Hector leads Ulysses and his crew on a Big Spree, and Ulysses doesn't notice at first that his companions has dwindled each successive time Hector passes around his flask.

Western Animation

 * The South Park episode "Die, Hippie, Die" has a large contingent of hippies arrive in town from Colorado's big cities for a massive 1960s-style music festival; they claim that the purpose of the festival is to stick it to all the corporate bigwigs and other "little Eichmanns" who supposedly run America. Stan, Kyle, and Kenny naively join in the "fun" at first, donning their best "student protester" outfits and learning to play the guitar. But as the festival drags on they start to become bored, and they realize the message of social activism preached by the college kids is, well, pretty much b.s. And on top of it all, all the marijuana smoke is starting to make them sick. They try to leave, but the crowds have become much too thick...

Real Life

 * This very wiki.
 * Drugs.
 * Sexually transmitted diseases.