Here is an easy do it yourself prank. All you need is a cube of chicken bouillon. You unscrew the shower head and mash up the bullion in inside. You screw the shower head back on and wait. When your unsuspecting victim turns on the shower head they will find themselves taking a shower in chicken broth.
The great thing about this practical joke is it completely harmless and it is adaptable. You could really use any substance that is water soluble and non-toxic. The only problem I see with this trick is that many people turn the water on to adjust the temperature before entering the shower and would catch on before they ever got wet. Course it would still be funny seeing them try to figure out why the shower smells like chicken soup.