I would write ups having the sub use Hour of Code or the Express Course. That way you are reinforcing the coding, and making it easy on a Sub with little or no experience. You could also plan some simple unplugged lessons, or an alternate lesson that is supplimental. I use the history of computing, or something related. Anything to keep the students from just wandering off to Youtube or something! Have the students lead the lessons....if you know you are out, make the expectation that the students will provide 10-minute "mini" tutorials on artist, app lab, web lab, or any other way to showcase talent, and teach the class something new! The sub just moderates the event!