Math Class To Self-Study?

Hi, I'm a Junior in high school. I just finished Calculus 3 (and got an A) last semester and I'm taking Differential Equations next semester. Also (if I get in to this free math program), I will be taking Linear Algebra and Number Theory with Cryptology this summer. Anyways my question is what math subject would be good to self-study (that I wouldn't be repeating much by taking the classes I mentioned)? Would Discrete Mathematics be a good idea? Also how would you recommend I self-study, just textbook and problems? Also please suggest good textbooks and other resources. Thank you