Found this idea on Pintrest.
How to make smashable homemade 'rocks'
mix 1 cup flour with 1 cup of water, add food coloring, dip cotton balls and cotton rounds in it, place them on parchment paper and bake until hard (about 300* for 30-45 minutes.)
In the sensory bin are the 'rocks' and some tools.
this is what a broken one looks like. Both kids were surprised at what they found inside. The textures contrast nicely: hard and crunchy vs. soft and stringy
They're pretty hard to break, which make it more fun.
Lula got a real kick out of this activity
This is a fantastsic activity for toddlers. They get to destroy something! It does make a bit of a mess, but it can be vacuumed up easily. We played a game with the colors we made. I asked him to smash certain colors. He was learning and having a blast.