Write five things that creep you out.
Describe the scariest place you've ever been.
What scared you when you were little?
Survey at least five people and ask what scares them.
List ten good, scary words.
Think about five things that you want. Pick one and write a bit about something bad that could happen if you got it.
Write a short description of a story that takes something harmless or nice and turns it into something scary, like vampire hamsters or ice cream that freezes people.
Create a monster. Use Photoshop, or old magazines, or paints to create your vision of a frightening monster...or a deceptively nice monster!
Did your monster/villain used to be normal? What does your monster want? What hurts your monster? What makes him stronger? How can he be overcome? What does he eat? What does he like to do? Where did he come from? Do a bit of 'freewriting' (with no concern for spelling or whatever) and sketch out your monster's backstory.
What's the story on your hero? What is his favorite book? What unusual talents does she have? What is he afraid of? What are his strengths and weaknesses? What does she want?
List several places that you love to go. Write a short sketch of a story about a scary, strange event in one of them.
Choose a moment in history, and create a scary story for that time and place. What if a giant, genetically-engineered cow showed up at the launch of Apollo 11?
Create and do a character sketch of someone who is compelled to do something odd or dangerous every so often...