If you have an instrument, you could perform/log your progression in learning a piece of music.
You could also do creative writing in another language.
You could design or draw a series of paintings or attempt to mimic famous ones.
I’m not exactly sure about the limits of what one can and cannot do but CAS coordinators are usually flexible about it.
Do a website on reviews on something you like with other 2 friends. Football, politics, films, fashion, whatever. Another advantage is that if you develop it for more than a month it can be your CAS project (group, more than a month and learning outcome 3 (which means that you have to create it yourself))