Extracurriculars for Astronomy

What are some good unique extracurriculars to do for someone who wants to study Astronomy at uni?

Look at the Astro challenge or https://www.bpho.org.uk/baao/round-1/
Also any physics supercurriculars like COMPOS or BPhO as astronomy is basically a physics specialization.

Just be aware that most top Unis are not that impressed with 'competitions', whatever your teachers may try to tell you. It tells them nothing about your individual interests within this subject, or your potential for higher level study. Do the extra reading, Moocs etc, and then discuss those new ideas and thoughts in your PS.

https://www.undergraduate.study.cam.ac.uk/files/publications/super-curricular_suggestions.pdf
https://www.ox.ac.uk/admissions/undergraduate/courses/suggested-subject-resources
- these official lists mention olympiads as suggested supercurriculars. (Deja Vu? I think I sent these links to you before)

'Most'.

I don't see why only Oxbridge would appreciate them while others would not.

Frankly - because they prefer 'a certain sort of student' and therefore from the sort of schools who do competitions. Their commitment to widening participation is very superficial.

How are olympiads wealth-gated? They're cheaper for schools to offer than further maths, aren't they?

Any Uni that uses this as a filter is adding an additional obstacle to those from disadvantaged schools where factors like cost already prevent them offering FM - its inherently favouring those from better resourced schools.

BAAO Astro challenge is free for schools to administer, and round 1 gives schools four free papers, so an under-resourced school could give the Astro challenge to everyone who wants to take it, and round 1 to the four best scorers all for no cost.
BPhO round 0 is £14 for a whole school (not exactly debilitating) and round 1 gives schools four free entries.
Even beyond the four free entries, further entries are still cheaper than a GCSE resit. Are universities that consider GCSEs inherently favoring richer students and better resourced schools who can afford to pay for them?