Open to suggestions.
How to teach a toddler the concept of zero. (The official version is better but got deleted off of YouTube.)
How to visualize transformations in the plane (Möbius transformation).
Here's a practice paper which introduces the topic of factorials, as well as combinations and permutations (counting techniques which let you compute the number of ways you can line things up or the number of ways you can choose groups).
Here's a practice paper which introduces the topic of complex numbers. It's just an introduction. There will be others which delve into arithmetic and applications.