Physics supercurriculars for Oxbridge

COMPOS follow along programme (if you're ambitious you could even try the problems from y12), quantum club (next year), IsaacPhysics (pro tip: register with your real name so the problems you solve might help you qualify for the SPC), self-studying for BPhO (recommendations depend on your background), self-studying for BAAO, maths supercurriculars (UKMT, learning calculus)

I've already done MIT's multivariable calculus course last year and I got a bronze in BPhO last year too, is there anything else I can try?

Sorry, I pressed reply too soon...
Anyways, what I would recommend is following your interest in a niche physics topic and doing research on that will look great as a supercurricular. This could work towards an EPQ, a project, or just a personal log you keep - but the main idea is to just generally consume university level content in you field, this could be books, podcasts, journal articles, lectures, etc...
This will also really help when writing your personal statement next year.
Best of luck x

Hi! I'm in Y11, and I'm starting to look at what supercurriculars I should do. I'm planning to apply for Physics at Cambridge, and I play the cello (I have a diploma) and I do swimming. Apart from that, I've got a few bits and bobs here and there, I'm doing the BPhO this year (round 0 is tomorrow, i got a bronze last year so hopefully I get past that). I'm also working on a completely new optimisation algorithm based off quantum hydrodynamics. What supercurriculars apart from these should I do to maximise my chances of (hopefully) getting to cambridge? Thanks a lot in advance!

Just to note that neither cello nor swimming are supercurriculars, they are extracurriculars and will have no effect on your application. The BPhO is absolutely a supercurricular as it relates to Physics. Heavily Maths- and Physics- focused extras are really useful too.
Another thing to note is that the Cambridge course is a Natural Sciences course. You can enter in through Maths as well, but it may be worth looking at which route works best for you.

Yes, if you look at the math courses available in years 2 and 3, you'll find a lot of theoretical physics courses that would prepare you for part III in astrophysics or part III in theoretical physics, both of which are under the umbrella of the part III in mathematics.

Just to note that neither cello nor swimming are supercurriculars, they are extracurriculars and will have no effect on your application. The BPhO is absolutely a supercurricular as it relates to Physics. Heavily Maths- and Physics- focused extras are really useful too.
Another thing to note is that the Cambridge course is a Natural Sciences course. You can enter in through Maths as well, but it may be worth looking at which route works best for you.