Egg cartons of all sizes are a GREAT way to introduce, practice & reinforce DOZENS of math concepts.... yes, pun intended!!
As I find resources, I'll add them to this page. Here are a few ideas just to get started.
You don't need plastic eggs... ANYTHING can be used to put in the egg carton to represent the eggs.
Also, you don't need to print copies of egg carton worksheets... you can draw your own cartons or use actual cartons to practice.