To begin with, I invited L to color all over coffee filters using washable markers.
Once the coffee filters were colored to L's taste, I handed over a small spray bottle filled with water. The idea here is that the water will cause the marker to bleed and the coffee filter ends up with these excellent patterns and mixtures of colors.
L spent a good amount of time exploring the spray bottle and attempting to squeeze the thing to get the water to come out. After a bit, she asked for my help, so we squeezed the bottle together and watched the colors bleed all over the coffee filters.
After they had dried a bit, we displayed them in our front window. It was gloriously sunny that morning, and they ended up looking like lovely sun catchers floating over the living room. L was even more thrilled when we went outside and she could still see them from the street.
This was a super easy project with very little muss or fuss. L was happy and engaged and covered in marker for an entire day...she's a bit of a sun catcher herself, really.