Right before a lecture as you are walking into the classroom, ask one of your classmates if they are "ready for the test today?" involve one more pal of yours in the prank who can authenticate your statement that there is indeed a test that day.
Keep a spool of thread matching your shirt and hide it in your pocket. leave a length of thread hanging out. when your victim will see it and try to pull it out. it will keep coming and coming!
Cover the hood of your victim's car with think plastic sheet, tucking the edges well under the hood so that it's not visible in first instance. then paint some funny graffiti or message on the plastic with paints. hide nearby to see your victim's face when they see the "damage."
ask a pregnant woman to take a pregnancy test for you, then bring it home and leave the stick in the washroom for your husband to find.
Find a small picture of a a funny creature ( monkey , donkey , baboon, etc) and use a glue stick to place it over the photo on victim's driver's license. then send them out for beer, or have some better excuse to send them somewhere where they need to show their id.
Go to the video store with the victim and have them pick out some movies they want to watch. then when you get home, have them go get some snacks or something and while they're distracted, switch the dvds inside the cases with some really lousy movies you know they'll hate. then laugh when they think that the video store gave them the "wrong" movies!
Get up early to unplug the toaster. if the cord is normally visible find a cord that looks similar, plug it in and hide the rest behind the toaster. besides not working, most toasters won't even lock the toast down when you push the pedal. just make sure you're there to watch your victim become frustrated as his toast keeps popping back up!
Take a brownie and form it to look like dog poo. place it on the grass of the victim's yard. as you and victim pass by, point it out to the victim, bend over and examine it. then grab it and put it into your mouth; or throw it at the victim. the victim will get a momentary shock until they figure out they've been tricked.
Put a potato in the exhaust pipe of the car. the car would not start. paint the potato black so that the victim cannot see the potato.
Put a cling wrap or any other clear plastic wrap over the toilet, between the bowl and the seat. do this at night so it is harder to see. then when someone goes to the bathroom, surprise! oh, and a scream! be prepared to be grounded!
Offer to make a sandwich for the victim. but don't remove the wrapper from the slice of the cheese. when the person bites, he'll get a chewy surprise.