Number magnets to keep track of kids you allow to leave the room. Use a cookie sheet with glass stones glued to scrapbook paper and a magnet. then trim in fun duct tape! (The cookie sheet magnet board is pretty sweet for multiple different crafts.
This is just something that I thought would be very useful to me as well as the substitute teacher. Everything that the sub would need would be in this area so it can be found. I would also probably assign a student to be the sub's helper for the day.