Fun for elementary Spanish. Even if it's not a required subject I think I would still try to incorporate it because kids learn faster and retain that information better than adults. Maybe have extra credit quizzes on it or something.
Idioms are a great way to incorporate culture into your classroom. Here's a list of 27 I use daily in my elementary Spanish classes. Mundo de Pepita: Spanish Idioms for the Classroom- Comprehensible Input in Context