Some good ways to think of writing topics include freewriting, listing and researching, according to DailyWritingTips.com. These methods all make you perform tasks that can spur you into a creative phase.
To use freewriting, simply come up with a quantitative goal (such as 500 words or 30 minutes) and write. It doesn't matter what the subject is, because it is an exercise to spark your creativity.
With listing, jot down any word or phrase that represents what you want to accomplish with the piece. Researching is similar to listing in terms of being a way to think of writing topics, but you read about a general subject and look for inspiration within the subject's background information.