This will sound weird, but I would love to re-take some math classes (trigonometry, calculus, whatever). I was always decent in math, to the extent that I took enough classes in high school for college credit that I didn't need to take any math in college, and I did ok, but didn't get everything I could have out of it. I chose not to take elective math in college, and my major and minor were both in the humanities, which I loved, don't get me wrong. But I think back to some of my high school classes and the teachers going through all the proofs for the different theorems and I wish I could do all that again and pay attention this time. Like, now that I'm an adult I finally understand what they meant when they said math teaches you how to think. I think taking more math probably would've helped me in law school actually.
I find learning math similar to learning language in a way - it's all about learning complex rules and being able to apply them instinctively. In a similar vein, I really enjoyed taking Formal Logic in college, so I would recommend that, or any philosophy courses.