Here is an interesting cause and effect art exploration for the easel. We put cardstock on the easel. (At first, we used drycleaning cardboard but some of the white paper does get removed. Cardstock works better.)
The children cut pieces of masking tape and placed it on the paper in various arrangements.
Then they painted over the masking tape. We put several shades of one color on each side of the easel.
When the paintings were dry the children removed the masking tape and they could see where the tape "resisted" the paint leaving white lines on their paintings. (We did try removing the tape as soon as they were finished painting but it was slippery and then left lots of paint in the white spaces lessening the effect. It was a much more dramatic experience when dry)