On days that my students aren't playing Math Facts Baseball, I give them a little bell work that requires explaining. It's quick, easy, and effective!
The examples below require my students to discuss place value (4.NBT.1). This really makes them think! As the beginning weeks of school tick by, the Writing to Explain problem cards will also ask them to explain their reasoning for writing numbers in standard form, words, and expanded form; comparing two numbers; and rounding to the nearest ten and hundred.
These short activities require metacognition. As students think about their own reasoning, deeper understanding is achieved. I first used this strategy last year, and it really made a difference!