Maths is the obvious one (run my own business, need to be able to do forecasts etc as well as for example yesterday I ordered some new tiles and paint as we're redecorating, so it comes in useful every day), but likewise English for puting together a good pitch, formulating ad campaigns etc, heck I've even written a book.
However realistically the choices of subject that I think have changed my career and income beyond where I'd otherwise be are drama (naturally shy, but drama helped me create a character so when I'm presenting I'm not Jane, but Jane The Business Owner, and she's not me so I don't feel vulnerable being up front and center) which gave me confidence, clarity of speech and the ability to hold attention when presenting, and then the other subject is IT, enabling me to build my own websites, start my first business at 18 when ebay was in it's infancy and grow several ecommerce brands since, to know how to use digital marketing as well as the basics of photoshop, office suite, to recognise how I can use AI and how it can streamline some of the processes I use in my business and so on. I think a lot of this is critical thinking and soft skill based rather than the typical 'hard skill' subjects in my case.