I saw this activity when I was knee-deep and lost in the Internet and I apologize to the children's art blog that had creating fish from leaves.
First I soaked some blue sidewalk chalk in water and Mister and Ladybug colored their paper using the wet chalk. I had them use the side of their chalk to make thick lines.
Next we took a walk outside to collect leaves and sticks that had fallen on the ground
I made a fish out of the leaves and they called out with excitement that I made a fish. They were eager to start their own fish. While Mister was playing with his leaves and created a sting ray and Ladybug followed.