The differences between the uses of future simple (will) and going to are very close, and they sometimes overlap. Here's a match-the-uses worksheet which compares the distinctive uses of both tenses. In the worksheet, there are two short quizzes with sentences which demonstrate different uses of will and be going to.
I suggest you pair the students and cut the worksheet in half. Then, each student can check his partner's quiz and learn more by explaining their answers to each other.