Extra activities can be extra-curriculars or super-curriculars. Extra-curriculars are things you do outside school generally unrelated to your study. You could volunteer, for example at a charity shop, or a youth organisation (e.g. Scouts). You could also take up a sport or hobby. These essentially tell the unis your applying to that you are an asset to the campus and student life beyond just your studies. Super-curriculars are study-related but outside the content you study at school. For this you could do some MOOCS (try EdX, Coursera etc), go to some lectures, read some books, listen to podcasts, do a project or some work experience.