When people break bones and get a cast they also get an opportunity to express themselves. They also get the opportunity to allow others to express something on their cast.
Writing on a cast has become a rite-of-passage for kids. It's less permanent than a tattoo, but it's just as cool. This page provides some clever things to write either to entertain or encourage someone wearing a cast.
Funny Things to Write on a Cast
Cute and Sincere Things to Write on a Cast
More Ideas to Say Get Well