[Arkan]

Afraid to get wet? Water fire extinguishers are always fun.

Or, you can sabatoge someone going into the bathroom. Get a cup and fill it half way with water, then you need to tape it to the top of the bathroom door so when they open the door, the cup tips over getting whomever wet. I've done that several times.

Or, you get someones jacket and staple the sleeves closed. As an added bonus, you can then fill in the arms with whatever is available (paper shreds, potato chips...etc)

The 'ol "Turn someones drawer upside down" trick.

Oh shit, forgot about this one. Get on someones computer and take a "photo" of their desktop. Now, make that photo their new wallpaper and watch them go nuts trying to get the mouse to work.

And this one is fucking classic but you'll need a few things to accomplish it.
You'll need:
A small bathroom
C02 fire extinguisher
Baby powder
and someone wearing pants who needs to take a shit.

......when someone's taking a shit, spread baby powder all along the floor at the bottom of the door. Now grab that C02 extinguisher and give it a blast at the bottom of the door. The force from the C02 extinguisher will propel the powder into the bathroom covering the pants of the person taking a shit. They'll get blasted with baby powder and C02 powder and you'll be having a difficult time picking yourself up off the floor from laughing.

.....i have a few more ideas if you're interested.