For the kiddies:
Faux Food ideas at Family fun. Hint, that's not a pie people, that's meat and potatoes.
Or how about using dye pellets and unscrewing the cap to the faucet and drop one in and watch the colored water flow when they turn on the faucet.
Or how about switching the drawers around on them?
OMG can you imagine coming into work and finding your cubicle like this? Thank God I have an office where this would be impossible because I know some of my coworkers would love to do something like this.
I think taping the phone's receiver button down so that when someone calls and they pick up the phone continues to ring is a good safe prank to do.
This one may come back to bite you when he gets you back but still a funny one to try. Put a rubber band around the spayer nosel so that when they turn on the water they get sprayed. Of course remember that you did this and don't spray yourself by accident.
You Smell Like Chicken: Place a bullion cube in the shower head. It's subtle enough that they won't notice but everyone else will.
And if you just too much time on your hands and you have an alarming amount of post it notes stashed away then this one is for you. Awesome!
Me? I'm going to tell Poppa that I've got an awesome prank planned for him, torture him all day with how awesome it is but the joke is I won't be doing anything to him except watching him squirm trying to figure it out.
Most of these ideas came from Martha Stewart website or Family Fun or just google.