Draw and explore with cornmeal - a fun sensory activity that is safe for even the youngest children.
A salt tray or salt box is a wonderful way to promote pre-writing skills and actually practicing cursive writing without the paper.
A wonderful go-to busy activity, not to mention sensory activity for toddlers, is to set out a tray for flour play! Flour play can be great for learning.
Maize Flour Writing Box - Teach Me Mommy