Those early finishers are extra tough for a sub to handle. There are plenty of things you can give students to do to keep them occupied. I make worksheets for those early finishers. This one is for Groundhog's Day and it costs .99 at my TeachersPayTeachers shop. You may find it here: Groundhog's Day Worksheet for Early Finishers
The worksheet has the lightly drawn outline of a groundhog, on which the students add details. There are also sample details they can refer to and add.
Now, I focus my work on art subs, but this worksheet can also be used by classroom teachers, art teachers and others.