Chromosomes for sure. You can talk about how they replicate and then line up in the middle of the cell during mitosis. Also, spindle fibers are important to mention as they pull the chromatids apart.
A mitosis short story could include the different phases. Like prophase where the chromatin condenses. Then metaphase with the chromosomes at the equator. Anaphase when the chromatids separate and telophase with the formation of new nuclei. And of course, the final splitting of the cell into two daughter cells.