Maths, English, Chemistry, Physics, Biology, Music, History, Geography, Medicine, Languages, Economics....
Those are the ones you might have thought of but then there are a load more which you might not even realise you can study:
Archaeology and Anthropology, Biochemistry, Biomedical sciences, Classics, Philosophy, History of Art, Fine Art, Oriental Studies, Theology...
There are so many courses on offer it can be tough choosing which one to apply for but it's good to decide in advance. Once you've picked you can use the next few months to really start delving into your subject.
For me personally Oxford was the dream and any college or subject would have done! If that's the case try to choose the one you're both good at and can imagine yourself studying intensely for 3-4 years. If you hate it after week one, your University experience isn't going to be much fun.
I was encouraged to apply for History or Politics by my school, as I'd obtained top marks but I knew I didn't know enough about either of them outside the curriculum, so I instead picked English.
Try to pick a subject you think you can build upon outside of school so show a genuine interest to the tutors.
Make sure you apply to a college which does your course, some of them aren't taught at all colleges.
Research all the courses here: https://www.ox.ac.uk/admissions/undergraduate/courses-listing?wssl=1