forfeit nuity
I was bartending in Kansas when we had a really fun night. We had some shots set up and the last one to down the shot and beer had to strip naked until the last person left. I lost so off came the clothes till everyone tabbed out for the night. That definitely started some interesting games going forward in that bar !
Not exactly a bet, but someone on another board posted that he went to an all male boarding school that had uniform inspection and if some or all of the uniform didn't pass, the student had to go without those parts for the day. Nudity was required for sleeping, swimming and most other athletics, so they were used to seeing each other nude.
More a challenge than a bet or forfeit but it was my first exposure in a non-nude-friendly place. It was at the end of a party in our village pub. Many of the ladies asked what was under my kilt and I was eager to show them. The landlord said he was going to light up for a smoke (not allowed then). I said I'd strip off if he lit up. He did and so did I. This is the photo he took.
There was a sequel. He showed the photo to the regulars the day after and before I arrived. When I entered there was a big cheer. I think I offered a repeat show.