You don’t need extra curriculars - Oxford does not care whether you do sport, music, or Duke or Edinburgh. They do like super curriculars, ie showing an interest in your subject beyond the school curriculum. This doesn’t have to involve anything fancy or expensive, it can be as simple as wider reading, listening to podcasts. The important thing is that you can reflect upon what you have done and integrate it into your thinking about your subject (a personal statement that reflects maturely on one or two books and shows real intellectual engagement is going to be better than one that just lists a ton of stuff without any analysis, however impressive the achievements). For the time being, just try to immerse yourself in your subjects as learn as much about them as you can. Ask your teachers for recommendations of further reading.
Tutors don’t care if you mix Humanities and STEM subjects as long as any admissions requirements are met. They will judge the application on its merits, not on what it would have hypothetically been like if you had done different subjects.