Switch the "Pull" and "Push" signs on a set of doors. Watch as people get confused trying to open the doors.
(Don't do this on fire escapes)

This prank works great on gullible younger siblings. Find a picture of the ugliest monster you can find and hide it in a dark closet where you can find it easily. Lead your younger brother/sister to the closet and show them the picture. Tell them that it was a photo of them when they were born.

Get three animals, sheep or pigs, and number them: 1, 2, and 4. Set them loose in your local school or office. People will spend forever trying to find the "missing" one, number 3!

NB: not really a good prank to do - think about the welfare of the animal.