Intro: Make a Confetti Trap
A confetti or glitter trap is a prank set-up which drops confetti or glitter on the victim from above. The cup of confetti over the door is a simple version of the same concept, but this method is much better. It can be set to go off on nearly any trigger (opening a drawer is a good one), anywhere with a drop ceiling. All you need is a little bit of thread, a funnel (which you can make out of a piece of paper), some glitter or confetti (three hole punches make an excellent and free source), and a mousetrap. A few plastic drinking straws can help too.
Step 1: Adjust a Ceiling Tile
Find a ceiling tile above where you expect the victim to be sitting. Get it out of the way. You could drill a hole in it, but there's no need for property destruction - nobody is ever going to notice if one of the tiles is a little bit cock-eyed. You'll also need access to the space on top of that tile, so slide the one next to it out of the way entirely.
Step 2: Drill a Hole in the Mousetrap (optional)
Drill a hole in the mousetrap right under the triggering mechanism. Measure the distance from there to the far end of the mousetrap. (You can skip the hole and measure the entire length of the mousetrap, but the mechanism will be more reliable if you drill it out.)
Step 3: Position the Confetti Funnel
Place the funnel somewhere where, when it is full of confetti and the plug is removed, it will dump confetti on the victim's head. Then slip a small piece of paper or card under it, so that it blocks the opening of the funnel. (This is only there right now to check that you can set it up properly.)
Since you may or may not be getting the funnel back ever, don't bother spending money on one. I just make one by curling a piece of paper into the right shape and trimming it down to size. You can use some of the rest of the paper to prop it up and keep it vertical, so the confetti will fall properly.
Once you've figured out the positioning, pull the card out. You're going to need it. Punch a hole in one end.
Step 4: Measure for Mousetrap String
You'll need two lengths of thread. One runs from the mousetrap to the card, and one from the mousetrap to the trigger.
First, figure out where the trigger string will enter the ceiling. This is a place where the positioning of the ceiling tile creates a gap for it, and is somewhere where the string can be relatively discreet. (You could even move another ceiling tile if nowhere on the one you started with is sufficiently concealable.)
For the funnel string, measure from the spot you just found to the funnel. Subtract the length of the mousetrap from the hole to the far end.
For the trigger string, just cut six or ten feet; you can trim it to length later.
Step 5: Position Mousetrap
Tie one end of the funnel string to the arm of the mousetrap, and tie the other end to a hole punched in the card. Tie one end of the trigger string to the triggering mechanism on the mousetrap, running the string through the hole.
Place the mousetrap on the ceiling tile, with the string dangling down in the spot you figured out for it earlier and the far end facing toward the funnel. Bring the card over to the funnel; you'll find it's most of a mousetrap-length shy of actually being able to block the funnel. If it's not, and can be placed under the funnel, then either your card is too long or you measured the funnel thread a little bit too long. (The measurement given earlier is inexact, since it ignores the length of the card and the knots.) Trim the card if it's too long, then take a bight of the thread of an appropriate length so the card just barely doesn't reach the funnel and tie a knot in it, shortening the thread.
Step 6: Set the Trigger
Tie the trigger string somewhere so that whatever action you want to have set off the confetti trap will tug on it, and doesn't have any length to spare. You may find a few straws useful in routing the string where you want it to go. Then trim off any excess thread.
The classic place to stick the trigger is the back of a drawer. Not having drawers in the desk where I set this one up, I opted for a computer mouse, placing the mouse too far back to be very useful and tying the thread to its cord.
Now, set the mousetrap. With that done, you should be able to carefully slip the card under the bottom of the funnel (its thread does not need to be under tension in this state). Fill the funnel with confetti or glitter. Now, when the trigger is tugged, the mousetrap will be set off and that in turn will yank the card out from under the tunnel, dumping it on your victim's head.
Put the ceiling tile you used for access back in place to make your handiwork as inconspicuous as possible, then wait.