"Completely surreal," chimed his blushing brother, Cameron, 20.

Their mother, Sharon, was stunned.

"There was full nude, semi-nude, interaction, you could watch, you could take part, it was just brilliant. Absolutely amazing. It's good fun," Sharon said.

Seedier and sleazier as night wears on

The van then sped to Patpong Road to sleazier shows.

The Johnson family chose to visit a sadomasochistic "fetish" club, where whips and chains hang on the walls.

While his parents and brother chuckled mischievously, Cameron allowed himself to be tied to a vertical rack and stripped, exposing his bottom.

His father, shrieking hysterically, followed the entertainers' lead and began lightly whipping his son in bondage, while everyone convulsed with laughter and Cameron cheerfully yelped in a mix of pain and delirium.

The young man then found himself tied up, in a tall cage, and dominated by a leather-clad woman as if Cameron was a submissive character in a Marquis de Sade novel.

"We offer a visit to a fetish club, which costs an extra 699 baht ($22) per person," said tour supervisor Toby Newbold.

"People may not want to visit such a place on their own, but feel more comfortable doing so in the comfort of a tour.

"I would say 70% to 80% of customers include the fetish club, which has been a real surprise for us.

"Most people are shocked to find it quite erotic, and some come out of their shell and want to participate."

'Safety and not being hassled' are sales points

Without extras, a Hangover Tour's basic price is 1,990 baht ($60), including two drinks, transportation to and from your hotel, plus a guide.

More than a thousand foreign customers have taken the tour since it began in 2011, according to Newbold.

"We expected the tour to be mostly young male customers, but the tour is very popular with couples and we have lots of groups of girls and women taking the tour," said Newbold. "We assume safety, and not being hassled, are a factor.

"This is a lighthearted look at Bangkok's nightlife. We just want people to have a good time."

The Hangover Tour doesn't duplicate the movie's dangerous adventures.

"In the movie, there is sex with a ladyboy, Stu gets a real face tattoo, there is drug taking and a young guy loses his finger," said Newbold. "So I would have to say our tour is a little bit more toned down."

"Crazy fun" but at what price?

Another Bangkok nightlife tour, Thailand Bucks and Hens, is similar.

Five Australian male executives in their mid-20s allowed CNN to accompany their Bucks tour, but asked not to be identified.

Their tour began in a scuzzier bar where girls hassled the five men for tips, confusing and embarrassing them when they were advised by their Bucks tour chaperone to give a bit of cash.

Their Bucks tour then moved to the same Nana Plaza bar where the Hangover's Brits were spanked.