Water color crayons. Shaved onto the paper with a sharpener, then sprayed with water and sandwiched between paper and ironed.
Use a pencil case to store sand and an unsharpened pencil to practice writing letters, names, and words. You could use the same setup for writing in the sand with your finger.
Messy Play in the Sink with Vinegar and Baking Soda
Such a fun idea!
How to make melted crayon art...