Take Your Cues from the Textbook
As you did the readings, what piqued your interest? Did you have any questions about what you read? Did you find yourself challenging any of the authors' assumptions, or thinking they were overly broad?
If so, you can use those reactions to build a topic for your paper. For example, if your history textbook mentioned John Brown's violence in Kansas as an example of pre--Civil War tensions, you could delve deeper into the topic of "Bleeding Kansas" for your assignment. Historians have written much about the Wakarusa War, the raid at Harper's Ferry, the Battle of Osawatomie, the Marais de Cygnes massacre, and guerrilla violence on the Kansas--Missouri border, so you would have plenty of resources available for your paper.
Pick Up a Magazine
Either online or in the library's periodicals section, look through the latest issue of a journal in your course's field. What articles or editorials jump out at you? Use their topics to generate a topic for your paper.
For example, pretend you're writing a paper for an education class. In the Spring 2017 issue of the journal Education, there's an article titled "Developmental sequelae among caregiver's stressful police contact among minority children starting school." There's a great topic: the academic impact of stressful police encounters on children.
Let Your Paper Do Double Duty
This is arguably the most commonly used strategy: letting your career aspirations and personal passions guide you to a topic. Maybe you have a relative who has just been diagnosed with a rare genetic condition, and you want to research the effectiveness of various therapies. Perhaps you're aiming for a career in high fashion, and you'd like to write about the evolution of a particular brand.
There are two concerns associated with this strategy that you should guard against. First, if you're emotionally attached to a topic, you may resist your professor's efforts to streamline or modify that topic. Second, writing papers should push you to explore new topics and exercise new areas of your brain. If you do write a paper on your specific career ambition, resolve that next time, you'll write on something completely different. Who knows --- maybe you'll discover a new passion!