I really should stop doing Sunday-style comics, as they always end up feeling unfinished. Anyways, the costumes in the first panel are from left to right:
Mark in green fur dye, a cheetah couple dressing as their opposites, A Canid trio as a Davis High School student, and a fox & wolf as Millie and Ozy, respectively.

And yeah, I cut corners on the fourth panel.

As for what Sam (as Peppermint Patty) and her friend are watching, it's a horror film about a murderer who kills his victims by a simple call on a phone.

I seriously hope this doesn't count as copyright infringement. I am an extremely poor folf and can't afford a lawsuit.