What’s That Smell?
This one is pretty simple. When attending a friend’s house party simply roam about their home, secreting things like tuna, shrimp, or any other product that will, over the course of a few days, turn into a foul and rancid odor. The trick is to put it places not easily found, or cleaned. You can unscrew the decorative end off a curtain rod and typically push something inside. There is also the option of drop tile ceilings. The choices are only limited by the amount of evil you wish to visit upon others. Within days the odor will be unbearable, and it is impossible to clean away a smell you can not find.
I should say however this may not be the brightest idea if someone has a violent seafood allergy.