Hahaha, you bet the mess adds to the fun! I'm always amazed at how kids can get so creative with their artwork, it's quite inspiring actually!
As for keeping the young'uns entertained, I must give credit to my Mom adjacent who's a teacher by trade. She has this uncanny ability to come up with interesting activities on the fly and keeps a repertoire of tricks up her sleeve for such occasions. Being surrounded by kids also exposes you to many fun ideas - I often find new ones simply from observing the little rascals!