Determine which quiz or exam questions were answered incorrectly by many students. (D2L quiz statistics can be helpful here.) Have students that answered the question correctly teach a short lesson about the question and concepts involved. This is particularly helpful for low-stakes quizzes, where material will be repeated on an upcoming exam (summative assessment.)
Assign each student a concept or procedure covered in the first half of the semester. Students then produce short tutorial videos, or text/image explainer pages about that concept or procedure. Grade, with directions for revision. Assemble the results as a study guide for a comprehensive final exam.