I think the most fun I’ve ever had learning anything was in middle school and early high school when I was teaching myself to read early-period Egyptian hieroglyphic writing.
The reason is that almost no one knew what I was doing, no one could help me, it didn’t matter to anyone but me, and I could just learn it without anyone else’s opinions on it — and it was totally useless.
Admittedly, I never got very good at it. I could maybe read 20% of a document with any understanding, and it was all arduous. Hieroglyphics are very difficult because the culture is so far removed and it’s an entirely different way of thinking.
But I had endless fun decoding those logographs* and straining my brain. I felt like an explorer on an uncharted sea.
*Egyptian hieroglyphics are more than logographs, but it’s too complex to explain here.