Firstly they will be understanding that GCSEs are overwhelming and leave very little spare time. You could talk about your maths club, I think that shows good commitment to your studies which is obviously an attractive quality. You could say you like reading ? Maybe read a few books around your alevel subjects and talk about those? That would show again a really good commitment and passion for your subjects, which they might even value more than extracurriculars which most other candidates have done