Description would be something like posting a picture and having people describe the scene. Dialogue could be setting a scenario and having people try writing dialogue for it. Characterization might include writing a paragraph or two from first person point of view to practice showing personalities. My idea is to give people opportunities to concentrate on different aspects of writing and for them to see how other people would handle the same set-up as well. It's easier to try new things when it's an experiment, not a treasured story that's been struggling to emerge.