Go to your bathroom and unscrew the shower head. Use a 9" Crescent wrench to loosen the shower head.

Place a few boullion cubes into the pipe leading into the wall. They come wrapped in foil. Leave them wrapped. You might even wrap them in plastic wrap so they do not disintegrate right away.

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. The water will dissolve the boullion cubes and make a bit of a stink.