Additionally, egg cartons make great seed starters. You can easily rip each section apart once your plants are ready to move outside. And you can plant them directly in the soil. The carton eventually breaks down and softens so the plant roots can grow right through it.
You could also use them for counting and sorting games. My children love to use them to hold their numerous collections. I eventually reclaim them when they start to get ratty and shred them up for our compost pile. They make a great "brown" addition to your compost. And if you shred them up, they compost quickly.