The first thing I used to get when I asked student what they were going to write about was the famous saying " I don't know" At the beginning of each year I have student use an Ideas book. This is their personal book where they can write their ideas for their next writing project.
I had my student keep this book in their desk so they could jot down an idea when they thought of one or when we came back from a field trip or assembly, etc.
The intent of this booklet is not to complete it in one session but for it to be a working document. It should be modeled, shown how to fill in the topic ideas and then using the web or the category style plan. The plans should be in point form but enough information for students to begin a piece of writing.
This booklet could be used as a whole class lesson or it can be used as an individual activity. If you use centers in your classroom it could be used as a center, to give students time to think topic and start their plan for that topic.