Cool things I've dabbled in that I wish people had told me about earlier:
-Learning a language: not as hard as people say. Even if you only learn a little, it's guaranteed to come in super useful unless you're learning a dead language like Scottish or something. But that's cool too.
-Rhythm Games: a once-popular but now recovering field. There's a growing network of Dance machines in America and Europe and other Rhythm Games like beatmania or maimai are starting to make appearances too. A low entry barrier, high learning curve hobby with a lot of fantastic music to discover, though it helps if you are already into Japan or Korea.
-Board Games: not much to say here other than if you haven't closely examined boardgames before, there's a lot more to offer here than monopoly or cards against humanity. Look for a board game meet near your home and try out some nice tabletop games with a complexity designed for adults. A great way to pass the time.
-Blockchain: regardless of whether you believe in cryptocurrencies, there's a lot of cool solutions offered by blockchain technologies. Though part of my interest may be due to the fact it's still an emerging field as of this post.
-Visual Novels: some of the greatest works of literature but most require a huge repertoire of horrible otaku knowledge and familiarity to really enjoy them. If you're already a trashy human being such as myself, you have nothing to lose by getting into these. Normal people: stay away. These will ruin your life. I suppose these are pretty mainstream nowadays what with the advent of "visual novels on steam".
-Mahjong: Just like people here mention Go, Mahjong is a pretty sweet table game that is overlooked by many in favour of it's more popular reinterpretation as Mahjong Solitaire. I'm still pretty new to it though.
-Amateur Radio: not enough of a scene for this in my area but still a really cool use of cool tech. Worth a look if you are interested in trying something new.
I've been considering some others, such as playing piano, cooking, digital art and studying philosophy, but I spend a lot as it is on my current interests and don't have a lot of free time right now. I plan to write stories someday (but likely through a non-paper medium).