I explain how to use past cartoons from The New Yorker's Caption Contest as a writing exercise. I have found that it is a way to allow students to be more creative with their secondary-language, in contrast to generic activities that may require authentic answers but that do not come across as very original compared to what their peers think of.
For this activity, the instructor selects a cartoon to display and students brainstorm creative ideas for captions. In this example, students submit their captions online, but you may also have your students write the captions in class.
I use Google Search to locate past cartoons from The New Yorker Cartoon Caption Contest