Jokes are a fun way to learn more about how a language works, as many of them rely on the subtleties of words and wordplay. This teaches students more about how to use the language, as well as hopefully raising a smile. Why not read a joke out in class and see who can translate it first?
(First read this joke!)
¿Cómo se pone una elefante en una nevera?
Abre la nevera, saca la jirafa y pone el elefante.