I’m not sure what a supercurricular is, but these are what I mentioned when I applied for chemistry:
-Not sure if this is feasible in your school, but my friends in the year above started up a science club for junior years (I helped out and did health and safety for experiments, and brainstormed for ideas)- this is more extracurricular, but it was a lot of fun, showed organisation/leadership/creativity etc, whilst showing that I was comfortable in applying my science knowledge and explaining it in an accessible way?
-I also tutored in learning support, mainly science but also maths and English
-I was chosen to go on this trip abroad (for languages), but I mentioned some environmental issues and solutions that China was considering at that point
-I emphasised the enviro stuff with chem knowledge, then talked about some eco stuff I did with the school including some Scottish parliament stuff
-can’t remember if this was on my personal statement, but I also volunteered for a biology PhD student at the local uni (just picking through marsh samples and separating critters found)
-I vaguely mentioned some less related stuff too such as part time work I’d done
I’m afraid I can’t quite recall more than that (this was about 5 years ago) but even as someone who didn’t get involved with olympiads etc I still had a fair amount to talk about 😊