Allow students to write test questions and model answers for specified topics, in a format consistent with course exams. This will give students the opportunity to evaluate the course topics, reflect on what they understand, and what are good test items.
Make a rough tally of the questions your students propose and the topics that they cover.
Evaluate the questions and use the goods ones as prompts for discussion.
You may also want to revise the questions and use them on the upcoming exam.