Decide if you want Ox or bridge, they weigh supercurriculars and personal statement differently. Oxford Biomed, for example, only use GCSE and BMSAT to shortlist candidates. Your supercurriculars from this point onward matter very little to your admissions chances. Cambridge, however, is more holistic.
Taking a gap year is also a weirdly popular idea for aspiring Oxbridge applicants that haven't thought too far into their academic journey. Supercurriculars often won't eat up too much of your time unless you're doing something big and there is absolutely no reason to not apply in Year 13 if there are no other circumstances and all you want to do is go to Oxbridge.
Summer schools are a way to show a level of interest in the subject you want to study and a springboard to talk about your learnings, but nothing more. They are often run by third parties using only the university's campus and name, and unaffiliated otherwise.
Decide where you want to go and what you want to study first. Biomed and English Literature are two completely different fields, and you will need to cater your supercurriculars and readings accordingly.