When teaching writing, I usually give plenty of prompts and brainstorming activities in an effort to decrease the chances that a student will say "I don't know what to write." But sometimes I want my students to come up with their own subject matter. I recognize that doing so can be challenging. My advice to students who must write something without any parameters is to write about what is dominating their thoughts. Every minute of every day we are thinking, but usually there are thoughts that continue to pop into our minds more frequently than others. These thoughts are fodder for writing.
Of course, allowing students to write about what they are thinking can be dangerous. Do I really want to know what a teenage male is thinking about most of the time? Don't I already know?
And yet, I am a risk-taker. Those thoughts may become wonderful pieces of writing. If nothing else, I will learn a little more about my students and who they are as human beings.
When reading my students' writing, I may step into dirty places where there are strange and offensive ideas. I may also enter into worlds of sadness or fear or even anger. But alongside this, I will also experience joy and excitement. Through their writing, I will enter into their worlds. I will see what they see and feel what they feel. And I will use what I learn to make their education fruitful. That is what dominates my thoughts.