Ivory Soap in the Shower Head
Go to your bathroom and unscrew the shower head. Use a 9" Crescent wrench to loosen the shower head.
Trim bits of Ivory soap and stick them into the shower pipe leading into the wall. Add as much as you can.
Screw the shower head back in place and wait for the next person to shower. You may want to use teflon tape to keep it from leaking.
Most people will turn on the shower and wait until the temperature is correct before they get in. They will come up with quite a surprise when they see they have a tub or stall full of bubbles.