A friend of mine years ago wanted to try something.
A little background: He and his wife had just gotten married. His wife admitted that she and her last boyfriend had been very "open" - MMFs, FMFs, sex in public, group sex, hidden cameras, etc. She said she dumped her ex because they were having a FMF and he came in the wrong girl LOL. She's always been into kink. She even made a sex video with her husband showing everything start to finish, knowing that he was intending to share it with me. She didn't mind, she got off on it. She even waved to me in the middle of the action
His idea for a birthday surprise one year was pretty elaborate. We never actually went through with it, but if we did it would have been the hottest thing ever.
They lived at the end of a cove, and their house/garage faced the street. 3-4 houses of next-door neighbors had clear views of their garage. And he wanted to make use of that.
His plan was to prepare the garage while she was at work, and surprise her when she got home. We would hang a sheet over the inside of the garage door (you'll see why later). We would also lay down some carpet remnants on the floor in the center of the garage. He would suspend some wrist chains from the ceiling in the center, and set up a padded bench underneath them.
When she got home, the garage door would not open because he had disabled it at the circuit breaker. She would have to park out in the street and come in the front door. He would meet her there while I hid in the garage. He would wish her happy birthday, start some foreplay to get her turned on, and then blindfold her. He would lead her to the garage and begin playing with her on the padded bench. At some point, he would chain her wrists so that she was restrained, not tightly just playfully. But she wouldn't be able to move more than 2 feet or so. He would then begin to undress her, cutting off her shirt if necessary because of the chains. Once she was naked, that's where I came in. He wanted me to continue playing with her while he video'd it. During that time, he would re-enable the garage door opener quietly, so that she didn't know. After some teasing, he would put the camera on a tripod and we would take turns playing with her, and eventually we would both fondle her at the same time. He wanted to see her reaction when she realized that there were 4 hands. Due to her past relationship, he knew she was ok with it. But at any time if she objected, we would have stopped.
After a bit of 4-hand play, we would take turns with her, and she (in theory) wouldn't know who was who.
And for the big finale, he would reach over and open the garage door while we were both screwing her. (remember the sheet over the door? No one would truly see anything, but she didn't know that) He wanted to see what her reaction to that was. He thought she would freak out at first but then be really into it. She would think she was exposed to the whole neighborhood while her husband and I were screwing her brains out.
Sadly, their relationship started to sour before he got a chance to try it out. Apparently he was not adventurous enough for her, and she had been having flings on the side. Ironically, I think if we had tried it they might still be together.