A happy boy's green exercise book would likely have a bright and cheerful green cover, with the corners and edges slightly worn and bent from being carried around in a backpack or held tightly in the boy's hands. The pages inside would be filled with drawings, scribbles, and notes, written in various pens and pencils of different colors. Some pages might have doodles of smiling faces, cartoon characters, or sports teams, while others might have math problems, spelling words, or science experiments. The boy might have decorated the cover with stickers, pictures cut from magazines, or drawings of his own. Overall, the book would be well-used and well-loved, reflecting the joy and energy of the boy who owned it.