Here are some great writing tasks that I use in the ESL classroom:
Using target language. Have your students write a short using as much (or all) of the target language that you specify. This helps the students use the language in the correct context.
Give the first sentence and let the students write what they like. This helps develop the students imagination and creativity.
Team writing. This writing game is a great team exercise, but it can be done as a whole class, too. Write a sentence on a piece of paper and give it to a student (S1). S1 writes the next sentence then folds back the sentence that you wrote, so that only the sentence that S1 just wrote is visible. S1 now passes the story to S2. S2 looks at the sentence that S1 wrote and continues the story with their own sentence. S2 then folds the paper so that only the sentence that they wrote is visible. S2 now passes the story to S3. Each student can only see the sentence that the student before them wrote. Award points for the most creative/funny story.