Here is an Author's Purpose presentation that will work great as a min lesson. Show the presentation to the class, and then print out the slides and put up as a bulletin board for the week or beyond.
I really believe in adding some humor to lessons so it's not just the same old, same old. I have added some bubble shout-outs from "students who are watching the presentation" just to add some lightness to the lesson.
Included in the 15 pages are:
Introduction to students
A page for each purpose
A page for each purpose example
A sample assessment page with three scenarios
A print out page that students can cut out each purpose to hold up for immediate assessment
Answer page for assessment