I have a large library of curriculum related movies with worksheets for analysis that I use as incentives. If we finish the required reading, writing and publishing for a particular subject, topic or literature assignment, I schedule a Movie Day.
Students are required to write a note to their parents explaining how the movie is related to what we are studying and give a brief summary of all the work they've done to earn it. I always read the requests before letting them go home so that the student is not demanding to see the movie or describing it inaccurately. Students who do not return the note to me go elsewhere and do a related reading/writing project during the movie.
If the students have worked really hard, I allow them to bring snacks to share. It costs me nothing, and the students work at a high level in order to participate.