CAS has many purposes. The IB Diploma Programme is academically challenging. However, students who can do nothing but study are not that interesting to universities or employers. Nor will they change the world! CAS is one of the main reasons that IB Diploma Programme students are so valued.
CAS should add fun and a different dimension to your schedule than studying! You have complete freedom about what you choose to do although your CAS programme should be a combination of lots of different experiences including:
things you already do now
projects that are creative
service that helps others
activities that keep you active and healthy
This video by St. John's International School in Belgium gives a bit more information.