/r/PersonalFinance for not-super-technical-finanace. I'm more familiar with PersonalFinanceCanada which has single-handeldy taught me how to think about money and finance. Can definitely get a bit toxic sometimes.
/r/Guitar is surprisingly small but still fun, and /r/Frugal is another favourite of mine. I also frequent the many FIRE subreddits but I don't think those are as popular as other forums about FIRE.
In a similar vein, Stack Exchange can be VERY interesting. I often casually sift through WorkplaceSE and TravelSE, though I supposed that is more entertainment than educational.