I used to read a lot in class, and the teachers didn't care because they were focused on teaching students that needed more help. I had a calculator I played with, and found things like 1111^2 = 1234321, and tried to understand these patterns. I discovered the Collatz Conjecture this way, began to learn about exponential functions, etc.

I also learned to draw probability trees from an explanation of the Monty Hall problem I read once, and I think learning that at a young age helped Bayesianism feel intuitive later on, and it was a fun thing to learn.

Second the Anki recommendation, but I'm not sure it's the most fun thing.

Writing fiction was something I enjoyed too, and improved my communication skills.