Most of the classes that Harry attends in the books are omitted from the films.
With the exception of the first classes that Harry attends in the "Philosopher's Stone" and most of the classes that introduce a new teacher (the first Defence Against the Darks Arts class from years 2-5, first Care of Magical Creatures class and first Divination class in Prisoner of Azkaban, and the first potion's class in Half Blood Prince) classes do not play a major role.
The invisibility cloak is used occasionally in the films, but it sees a lot more usage in the books.
There are many characters that play a secondary or even active role in the books, that are completely omitted or mentioned only.
In the other films, almost all mention of house points are omitted (except for two instances, one in Prisoner of Azkaban where Snape takes 5 points from Gryffindor, and the other in Chamber of Secrets, when Professor Sprout gives 10 points to Gryffindor).
In the films, house points play a major role only in the Philosopher's Stone.
Peeves, as depicted in concept art for the film adaptation of Harry Potter and the Philosopher's Stone 1.