Okay Then her'es another Adult one - Again you should excercise caution and common sense and be aware of the law for your particular area / country / state (Public Decency laws do vary), and these will depend on how far you allow this to go. You have been warned!.
Okay, you need eight volunteers, Ideally Married Couples or "Good Friends" - ie four male and four female!.
Produce blindfolds and large black bags (uK Bin liners are ideal!) tell them that the idea of the game is for the men and women to each get into a bag and swap clothes with each other - using the bags to preserve their modesty! the first couple to be wearing the others' clothes is the winner!.
Get members of the audience to make sure the blindfolds are worn corrrectly. Right, tell each person that you are now going to give them each a black bag to step into. At this point you substitute 4 of the black bin bags that they saw earlier for clear ones - give those to the Ladies / Men (Depending on the function). Now you have one sex changing in black bags - the other in clear

.
Again - be very careful how you approach / run this!, and don't allow it to go to far, Usually Bra / Knickers in the case of females / Boxers in the case of males is legal. By this time they have usually got their own suspicions as to what is going on by the reaction of the Audience!.
BTW:- If you are cautious about doing this (I only do it at certain selected events!) then the game using all black bags is equally hilarious (and far safer) especially with the volunteers trying to change whilst blindfolded.