I've been wanting to set up a cooking class (this would be pretty difficult) and also a kind of film club.
We'd watch a movie then discuss it, talk about it, write about it.
Do you do camp in the holidays? That's great fun.

Newspaper/magazine can be good if you can keep the kids focused - I tried it for an after school activity but they just copied and pasted from naver and wrote about EXO - it didn't go well.  You could try for a regular output if the school will allow you to print it, and send one to each homeroom.  Get them to write and take photos about themselves and the school. Topics school activities, photos and comments (upcoming and recent), reviews of the lunch menu, profiles or interviews with teachers and students.  Don't let them write about their fav k-pop, it's done to death.

Same with a movie - try to give them some strict parameters.  My grade 6 girls did a short film about school bullying, which was great as I didn't prompt them to do it - so you can do topics relevant to school life - the principal might appreciate it. Get your CT to make some Korean subtitles for the other classes to watch and enjoy during homeroom.

Thanks for the awesome ideas everyone!  I really love the movie/video ideas.  These would be exactly what the principal is looking for in order to show off our school to prospective students! 

I definitely have a plan to assign certain topics for the videos and news articles, but motivation will definitely be the biggest obstacle.  I will probably have to do a lot of heavy editing and work myself, but it'll be worth it if the principal is satisfied.