Answer:
You can unscrew a shower head and put lots of a sticky candy like the sweet sticky gummy life saver in it and screw it back on and someone will go in and when they come out they will be sticky and go back in to get it off and get even stickier its funny!
Also, you can superglue a coin on the ground somewhere obvious and noticeable. Keep looking! It's funny when they can't get it off the ground!