This prank Instructable is easy to set up, and works well. It can be set up in a minute, and can be left unattended.
I did this. She was not amused. I LoL'd.

Step 1: What You Need

You will need:
1 balloon or rubber glove, preferably at least 1 foot across when inflated
1 stapler (with staples)
Tape OR string and thumbtack
Small candies, mini marshmallows, confetti, or whatever you can fit in the balloon/glove

Step 2: Setup

Place a sign on a door that opens into the room.
The sign should refer to what is in the balloon.... For example:
RAINDANCE! for some water
Taste The Rainbow! for Skittles
PARTY! for confetti
Etc.. Use your imagination!
Stuff a bunch of whatever you want to rain down on your victim into the balloon or glove.
Inflate it to about 1 foot diameter.
Put a couple staples in the top of a door. Only put one side of each staple in the wood, so that you can bend the free leg up to point at the balloon.
Tape or tie the balloon just above the staples.

Step 3: Bring in the Victim!

All that's left is to get your victim to enter the room.
You should try to get it on video... For future lulz.

NOTE:
Be careful about what you put in the balloon. Think about safety. Be sure no one is going to get injured.