Here is a short and effective ESL writing exercise. It helps students learn to develop an acute awareness of the vocabulary choices they make while writing. I found exercise on a website called The English Teacher.
ESL WRITING EXERCISE
Objective: ESL students write a six sentence paragraph. They cannot repeat a word, including contractions and articles.
Rules: The paragraph must make sense and demonstrate standard principles of a well structured paragraph (i.e. topic and unity). It should not be a simple collection of unrelated sentences.
Benefits: Students become aware of vocabulary choices and their word weaknesses. It might also be a little fun.
Give it a try and let me know the results.