Any supercurriculars in mind? Let me know!

Supercurriculars can be as simple as reading a few books! If you're interested in Psychology, I can recommend a few:

The Man Who Mistook His Wife for a Hat by Oliver Sacks - I think this one probably everyone reads but it's a really interesting and accessible exploration of clinical neurology that might be a good jumping off point for further reading if it interests you

Bad Science by Ben Goldacre - not a Psychology book but still highly relevant to Psychology and a lot of other science - it focuses on media misunderstanding of research and poor research practices in a really accessible and entertaining way. I think its a good intro to scientific critical thinking. If you enjoy it then The Doctor Who Fooled The World by Brian Deer might also be interesting to you, it's about how the author uncovered the MMR vaccine scandal. Not necessarily directly related to Psychology (though not not related I would argue), but still interesting as an exploration of science gone very very wrong.

Zero Degrees of Empathy by Simon Baron-Cohen - not recommending this because I agree with what he says in it (I really don't) but it's still a pretty interesting read, and given the slightly controversial nature of it there's probably a lot to talk about if you discussed it in a personal statement

For any subject, the Oxford University Press Very Short Introduction series is great, they have loads of books that provide an introduction to a bunch of topics across loads of subjects. An easy way to learn a lot about something new and get some ideas for further reading.

Aside from that, have a look at free online courses (e.g. the Open University have some free online courses, Harvard have online courses you can audit for free, and I bet there's loads more), see if any unis near you have talks for the public that sound interesting. If you're in the UK find out what your area's linked Oxford/Cambridge college is and see if they do any outreach events for potential applicants in subjects you're interested in. Oxford also run a course called UNIQ for year 12s during the summer for people from widening participation backgrounds in the UK, so if you're eligible for that I highly recommend applying. Also look into entering any competitions relevant to your subjects (e.g. the UKMT maths challenge, the British Biology Olympiad etc. if you're in the UK). If you do well that's something else to mention that can demonstrate a passion for a subject when you apply.

Sorry if you're not in the UK as some of this is UK specific but hopefully at least a few things are helpful!

Read a few books off Oxford's reading list for your subject and look if there's any free online university courses you can do (they're good because each one only takes a few hours and you can massively big them up on your personal statement lol)