Unfortunately, I have seen many of our fill-in teachers who seem to think that they are just “warm bodies” occupying a classroom when the regular teacher is absent. Those teachers spend the periods playing Hangman with the kids and rarely follow a lesson plan (even if one is available). I always make sure that I do my best to discover exactly where in the curriculum each class is at when I go to substitute teach. If a textbook is involved, I make sure that only one page is completed per hour- (or, in this case, 50-minute-) long lesson.
And I ALWAYS mark any work the students do in class, even if it is simply writing the date as posted on the whiteboard. This allows the regular teacher to pick things up in the proper place when they return without suddenly finding themselves a few days behind. If I am organized, I still have time to do something fun in the classrooms that may be a bit out of of their regular routine. I think that is why the kids remember me from previous cover lessons. I had a few students today who said they remembered me from similar appearances at the school four or five years ago!