Q: What are some easy themes to use?

Okay, so I have started a collaboration channel on YouTube and we have a new theme every week. I am the leader of the collab channel and it's me that gives out the theme for each week and people put in video which is related to that theme. The problem I have is that I am only a few weeks in and I am already stuck on a new theme.

I've been thinking for a while and I've decided I am going to have two subjects that the collabers can choose from. Eg, Music or Movies. (They would pick one or the other and do a video on that certain thing) And I would kind of go into detail of what they can use like it could be favourite music, a lip sync video or you singing to recording your own movie, play out a movie or talk about the best and worst movies. What do you think?

The themes the collab have already done are:

Introductions
Music
Movies & a special fact about them.

What else could I have? What two things go together?

Thanks in advance!