I just recently pranked my housemate. Similar to this, but a little more subtle.
You unscrew the shower head and put chicken bullion cubes in it (2-3) works best.
After the shower begins to heat up the cubes they start to disinegrate, making the shower's stream broth instead of water.
Its not enough of a shock for most people to notice right away, but they will leave the shower smelling like a cup of ramen.
Koolaid works too, but it all shoots out at once before the person gets in the shower and gets comfortable.