Sophie from Roubidou shares a tutorial at Sew, Mama, Sew! showing how to turn a button-down shirt into an itty bitty onesie. This is SUCH a great idea!! You can make them for boys or for girls, depending on the color and style of the button-down shirt you start with. Get the tutorial.
[photo from Roubidou on Sew, Mama, Sew!]