Pinterest that I wanted to try with Mason (originally from here). It looked easy enough, and ever since the bathroom activity Mason has really been into painting. The premise of this activity is that you dip an old plastic card (gift card, hotel key, etc.) in paint and drag it along your paper, or in this case, empty cereal box. And then you are a world class artist. The end.
Update: I realize that I don't have a picture of the finished product. Just trust me. It's pretty amazing.