Harmless and fun balloon prank. Balloons pour into the room when the "victim" opens the door they get a birthday surprise.
Step 1: The Application Of Streamers
Attach some streamers across the door frame of your child's/friend's/victim's bedroom with enough slack that you can stick a balloon in between the door and streamers. We chose Spiderman but you can use any streamers you like*.
Step 2: Blow Up Balloons
Blow up the balloons so they are not too big (or they will rip apart your streamers before your plan comes to fruition) or too small** (or they will fall right through the gap between the door and the streamers before blah blah fruition). We chose super-fancy imported "Ballons". Twenty of them seemed to be plenty for a soon-to-be four year old.
Step 3: Apply Super-Fancy "Ballons" to "door-streamers"
...Umm...Apply the super-fancy whozimawhatsits to the thingamadealie.
Step 4: Wake Up First. Then Wait.
If you want to get full enjoyment of this one waking up before your victim is the price. This works best while said victim is still slightly groggy. As soon as the door opens the ballons rush in and who wouldn't want to wake up to that at least once in their life. Guaranteed to make any kid's birthday just a little bit cooler.