Extra curriculars suggestions for a future medical student in high school?

What are some ECAs that would be beneficial for an IGCSE highschool student (year 10) wanting to go study medical?

The only thing I've been doing that I feel matters is piano (been doing it for seven years and given 6 Trinity exams for it, so that has to count for something right? - but is it relevant to what I want to study?)

Piano is a great thing to do in general, but medical schools really aren’t very interested in extracurriculars per se, as whilst being a great musician is a wonderful thing, it isn’t going to make you a better medical student.
Year 10 is very early and in general they aren’t very interested in what you’re doing before year 12, maybe year 11.
For now, I’d recommend you focus on getting the best grades you can in your IGCSEs, and join extracurriculars that you enjoy, rather than specifically for medical admissions.
Once you’ve done that, start reading around medicine: finding relevant books, articles and podcasts for example, as these are really useful for your personal statement, but mostly if you’ve done them in the year or two before you apply.
When you get to A levels, apply for work experience in a hospital - this is the only really non-academic thing they’re interested in and it’s really valuable for your application.

Overall, don’t worry too much for now, just work hard at school so you get good grades and are in a good place to start your A levels, nurture your interest in medicine and start hardcore preparation once you get to year 12. Good luck, you’re very well prepared if you’re thinking about this now so I’m sure you’ll do brilliantly.

UK medical schools are slightly less interested in "generic" extracurricular activities (although these can be useful ways to develop examples of particular skills you will be needing to showcase in interview) for at least shortlisting purposes, as they have extremely transparent shortlisting criteria which they all publish on their webpages. Normally this shortlisting for interview will only consider a) checking you meet academic requirements through having the right A-level subjects and being predicted appropriate results in them, b) that you meet their work experience requirements and then c) whatever their main shortlisting methodology is - which will usually be UCAT/BMAT cut off scores, and some may also score GCSEs in varying ways (but others may just have minimum GCSE requirements looked at under a) and not scored at all otherwise).

So first and foremost you need to ensure you meet the academic requirements, and then ensure you will score as highly as possible on whatever the medical schools you have applied to shortlisting criteria are. As different medical schools have very different shortlisting criteria, you should also not assume that they are all the same and just pick medical schools to apply to arbitrarily. Someone who is a very strong candidate for one medical school against their shortlisting criteria may be a completely non-competitive applicant for another against the different shortlisting criteria.

For the time being, for UK medical schools, as you are doing GCSEs you should just focus on doing as well as you can in those. Anything else can be started once you begin your A-levels.