I would teach the students about the phases of the moon as it is a phenomena that they can see every evening if they look out.
I like the idea of reading them a story which introduces them to the moon and its different phases in a fun way and also that it can link to other parts of the curriculum. I also like the fact that students are asked to observe the moon themselves over a period of time and that they can conclude that we are all looking at the same moon. I would also encourage them to ask family living in other locations to send them pictures of the moon.
At the end of the month when all the pictures are displayed it is a lovely way of discussing how the moon is changing each week and introducing the names of each phase with opportunity to add extra phases in if students are particularly interested. I also like this video https://www.youtube.com/watch?v=wz01pTvuMa0 as a way to explain why the moon looks different each night.