Get a toy baby doll, some diapers, a stopwatch, and a blindfold. Once all your guests have arrived tell them that the baby needs his midnight diaper change but the lights are out. So you are going to have to diaper the baby in the dark! Let each guest have a go at diapering the baby blindfolded.
The trick is to hand the guest the diaper upside down or inside out so they spend precious time figuring out which side is the right side up. Some of them might not even bother to do that and put on the diaper all wrong! The guests who does it correctly in the fastest time wins. It is great fun to see each guest fumbling in the dark with the baby and the diaper, and a nice photo op, too!