La hija del sastre by Carrie Toth and Carol Gaab is a great read for level 3 and 4 students! I personally really love teaching this novel as the anchor text for a large unit about the Spanish Civil War in Spanish Fluency Matters.
Idioms are a great way to incorporate culture into your classroom. Here's a list of 27 To use daily in elementary Spanish classes. Mundo de Pepita: Spanish Idioms for the Classroom- Comprehensible Input in Context