These are the best pranks that I know.
Hopefully you'll enjoy them too.
Take 2 Oreo cookies. Open one of the cookies and scrape off the cream. Replace it with toothpaste. You will have one normal cookie and one that has been tampered. Offer the toothpaste Oreo to a friend. Enjoy your Oreo as your friend munches in misery.
Then offer them a glass of water to be nice.
Turn Me Over
Take a smooth rock. Write with a skinny marker on one side the words "Turn me over." Then turn it over and write "Thanks turn me back over so I can prank someone else."
Ah the classic cereal switch prank. If you don't know what that is it's easy. Get two cereal boxes and take out the bags. Then switch the bags. Your victim won't get the cereal they were expecting.
The Sink Soaker
This one's a classic as well. Put a piece of transparent tape on the spray faucet so it's pressed down. Wait till your victim washes his or her hands and it will spray in their face.