Magnetic number fishing, they add what they catch! As well as addition, develops hand/eye coordination, fine motor--I love this for magnetic letter fishing too! When they catch a letter, they say its name and sound.
Silky soft play dough for kids using 2 ingredients: 1 cup conditioner (cheaper the better), 2 cups cornflour/cornstarch, glitter (optional). I was just thinking about affordable gluten-free play dough.
Celery flower art is another great Mother's Day idea or just a simple art project for the classroom. I think the idea of using food to create art would be an interesting project to bring to the classroom and have students work with and experiment with.