Here's an idea for all those thin cardboard boxes:
Supplies: Upcycled cereal boxes, strong scissors, a ruler, white acrylic paint, paintbrush, thick and thin black Sharpies and colored marking pens
Measure and cut the cardboard to 4 x 6 rectangles
Paint both sides of the cardboard white (it might take a few coats). You can also enjoy a lollipop while you're doing this.
Let it completely dry
Draw a picture on one side with a Sharpie
Then color in with marking pens
Here's the boy's version…
Of his new BFF, Beans the dog
Draw a line dividing the card in half on the back with room for a stamp (29 cents for postcards) and address.
You've got mail– and one of a kind postcard art.