Any solute will work. Sugar, salt, alcohol. I'd simply use really watered down gatorade. It won't lower the freezing point that much, but it may give you the extra few degrees you need.

EDIT: Just to be clear... this won't magically give you another 10 degrees before the solution freezes. Maybe a degree or two. However, colligative properties depends on the number of PARTICLES in solution, so if you REALLY needed to, you could mix sugar and salt and other edible salts to get the greatest effect. This will work to a point, but you just can't expect that much.