Learn a programming language. (If you already know one, then learn a harder one. If you know php, learn java, java learn C++, C++, learn assembler, etc.)
Install linux and *use* it. (If you're already using linux, install arch / gentoo. Make sure to compile the kernel)
Take CS AS/A-level, as an extra. (If your school doesn't offer it, take it self study)
Make something big, as a project. (could be used for EPQ)
Study an open university course in computing. (They're currently revaping the curriculum to focus on IT, so they might not be offered any more)
Start a programming / computer science club. (focusing on games will help attract participants)
Along with those, you could pretty much anything mathematical. My local university has a maths club, which is help once a month. If you want to do this, you could take F. Maths (self study, if nessecary), or even STEP. There a lot of maths competitions that you can enter.