Bloody Mary
Perfect for a slumber party, "Bloody Mary" allows you and a few friends to call upon the spirit of an evil murderess to present herself to you in a mirror. To play the game, go into your bathroom, either alone or with a friend or two, and close the door. After you turn off the lights, chant the phrase, "Bloody Mary" three times in a row. On the third chant, you may see a face in the mirror looking back towards you. At that point, you may feel inclined to flee the bathroom. Try to stay in the room as long as you can; whoever stays in the room longest wins the game.
Scary Story Contest
Before you begin the game, write on slips of paper or index cards phrases that typically start a traditional scary story. Examples include "It was a cold, windy night..." or "As the fog rolled in towards the abandoned mansion..." Write enough index cards for every person who plays. When you're ready to play the game, turn off all the lights and light a few candles. Assemble everyone into a circle and have the first volunteer pull an index card out of a hat or bowl. The person then has to come up with a scary story within one minute of reading the index card and tell the story to the group. Each person in the group takes turns making up a scary story. Whoever tells the scariest story, eliciting the most screams, wins the contest.
Nighttime Scavenger Hunt
If hosting a sleepover, first decorate your backyard with some fake tombstones and cobwebs. Then, come up with clues that will lead your group to a destination that unravels a mystery you'll create. You can write the first clue within a letter that tells the group a scary story, such as a murder. Phrase the clue in a clever way, like a riddle. For instance, if you want the group to find the next clue under a flower pot, write something like: "I sit in the sun and bask all day, and hold something that will wilt away." Place each clue near an object that the previous clue indicates. Equipped with only a flashlight, the group has to find the remaining clues until they end up finding the murdered person---you. When they are close to finding you, pop up out from behind something and scare them.