I challenge my students with prompts on their weekly sketchbook assignment. This calendar lists a semester of assignment ideas that not only tell them what fun thing to draw but also what medium and technique to use! If I left it up to my students, some would draw the same types of things over and over. This forces them to branch out and face new challenges.
Examples: Draw your favorite toy from when you were little with pencil using gradual blending, Draw your spirit animal in ink with cross hatching, Draw what would be inside your favorite fictional character's sketchbook with a medium and technique that makes sense for him/her/it.