For curious people with active minds, some activities are obvious: Reading new books, seeing new movies, trying different food, etc. But there's an echelon above the obvious where you can take learning to a higher level without necessarily investing more time or money than on the ordinary stuff. The key, at least in my experience, is randomness: An inquisitive mind puts things together all by itself, so all you need to do is feed it. And rather than feeding it predictable material in order to build up a specific knowledge base, the best results seem to come from persistently exposing yourself to the unknown and unexpected.
In no particular order...
1. Watch movies, TV shows, and news reports in languages you don't know without subtitles.
Unless you're a linguistic savant, you're not going to actually learn another language this way, but you're going to learn a lot nonetheless. You start to pick up on things about other cultures that would not otherwise be obvious, and with a large enough spread of cultures, you start to learn things about humanity in general based on the similarities while being able to marvel at the drastic differences in style and approach. Both the differences and similarities can be shocking. You start to find that some other culture you had never really saw that way is shockingly similar to yours, while another is more different than you'd imagined.
If you plan on watching a foreign film or anime, watch it in the original language without subtitles first and try to figure out what's going on, then watch it again with subtitles. Do this often enough and nuances will become apparent that would otherwise have gone right over your head. If you have time, watch it in every available language so you can get a sort of cultural parallax perspective on the material. And don't be too snooty to watch the dub tracks in addition to the subtitles: The way a dub actor says something in a different language can yield clues about the cultural attitude toward the material. Especially amusing: German-dubbed anime.
Youtube is especially useful for watching foreign news reports and entertainment shows. Also, since the decline in the quality of Netflix streaming content, there has been an ironically enriching profusion of foreign content - not only movies, but TV shows and documentaries, although usually in English or with subtitles. Still, this is interesting stuff if you're someone who can't afford to be a world traveler, and (like me) doesn't have the life skills to just wing it and go. And because you pick it up at random, it's not as tedious as trying to pursue a logical course of education about other cultures that would involve actual work. It's intelligence on the cheap: Soak up so many random things that patterns emerge all by themselves.
2. Seek out material about one culture made by a different culture.
As Americans, we're accustomed to a lot of movies with American culture idiotically grafted on to foreign or historical societies, but because of that it's a lot of fun to find the opposite: Movies about this country made by and for another country, or by one country that isn't the US about another country that isn't the US. TV shows with that kind of theme are rarer, but still exist: For instance, it's always fun to see Americans depicted in Canadian TV shows - we tend to range from boisterous (but good-natured) assholes to invading barbarian hordes raping and pillaging the decent folk of the North.
It's always a titillating discovery when you run into some attitude between one country and another that you had absolutely no idea about, hinting at histories you wouldn't otherwise encounter unless you specifically read tediously comprehensive, multivolume historical works about the countries involved. Things that are footnotes or completely nonexistent in US-derived world histories can be a very big deal in a given country, with all the drama and cultural gravitas that Pearl Harbor or D-Day have for us. No matter how much you learn about it, you'll never stop being shocked at how big history was and how much stuff happened in it, and how massively diverse the institutional players and identity groups in it are.
But getting back to the subject, watching something about one culture by another culture teaches you about both, and is very fascinating as long as it's not purely formulaic. I wish there was more such material than there is, but sadly most other countries' pop cultures are actually more insular than ours, largely because they have relatively rigid divisions between intellectual content and entertainment while we often tend to bastardize the two with mixed results. So you'll definitely find stuff from China or Korea about Japan, and stuff from Poland and Russia about Germany, but beyond those really intense historical relationships, not so much.
Hollywood churns out bullshit pseudo-historical movies about other countries and cultures by the bushel, but foreign counterparts in the same vein are harder to come by and a lot more conspicuous because of it. The Russian movie about the rise of Genghis Khan, Mongol, was pretty fascinating, although Mongolians apparently detested it for the liberties it took with their history.
The closest thing to the Hollywood approach I've seen was a Japanese movie about ancient Roman baths based on a manga, called Thermae Romae, where Japanese actors played Romans in the same way Hollywood typically has Northern Europeans or Americans imitating British accents playing Romans. It was written by a woman who clearly enjoyed thinking up excuses to show a lot of naked dudes, but it's still mainly about comparing Japanese bath culture to the ancient Roman bath culture, and also contrasting the forwardness of Roman/Italian culture with the introversion and politesse of Japan. Anyway, it's fun to find material about one culture that isn't us from another culture that isn't us.
3. Watch news programs that are totally irrelevant to you and everything you currently care about.
I don't mean the local US affiliate of one of the five identical news conglomerates in this country dishing out the same boilerplate bullshit about another city than your own: I mean listen to news stories about a corruption scandal in the Belgian parliament, South Korean tax policies, South African education policy, etc. etc. Everything is ultimately connected, and you'll learn just by soaking in a wider context. Even if your eyes roll up in your head some of the time, other times you'll realize, "Hey, there's something I didn't realize here. I never knew that the __ government was so dominated by the interests of the ___ industry. Didn't know that __ society was so divided on this issue. Wow, there's some real douchey policies going on in the __ Ministry of the ___ government." Etc. etc.
Also, you find yourself understanding things that allow you to run intellectual rings around people who otherwise have deep but narrow knowledge. An expert in paper knows nothing about how novels are written, and an expert in literature may not understand the cultural background of the material they read and interpret. Someone may tell you all about the parts of a machine yet have no understanding of how it works, why it was constructed, or what it signifies. It's not the details that make for knowledge, but the space between the details - the relationships among them. That is the difference between someone who knows a million disparate facts with zero understanding and one understands much from little specific information - the difference between trivia and wisdom.
4. Go to random places without advance knowledge of what is there or what is along the way.
Pick a distance, then randomly pick a destination at that distance and go there without attempting to learn anything about it in advance. It doesn't actually matter where you end up, as long as you're reasonably careful to avoid wandering into a gang war zone or a drug cartel route or something, if those are real dangers in your area. The point is the process of discovery, the journey. And it doesn't have to be some epic road trip either: Park somewhere and take a walk for a few blocks. Or drive a few minutes up a road you've never been before. Take a long road to its end. As long as you don't turn, you can't get lost - you can always just turn around and go back.
We lose a lot of the impetus for this kind of exploration with the generic sameness of urban sprawl, but even that can't really hide the differences that exist in places. Go far enough, and the air changes; the foliage changes; the quality of the sunlight in otherwise identical weather changes; and the land behaves differently. Even in your own city, no matter what size it is, there are corners you don't suspect - perspectives you wouldn't have guessed if you weren't walking or driving right there. There are scraps of perfection hidden in plain sight. You can be a wanderer without going that far - travel for a human being is not really measurable by distance: A billionaire can flit between continents on their private jet and experience nothing of value, while you can simply turn a different corner of the sidewalk and find another world.
5. Crash college lectures at random.
This isn't entirely possible, actually: You know what building you're in, for instance, so you know if you wander into a lecture in the Physics building that it's probably got something to do with physics. As long as you stick to the big lecture halls, you'll probably avoid anything potentially awkward where it would be obvious you don't belong - the more specific the subject matter, the smaller the group. So pick a building and crash the big lecture hall, or roll dice or something to really make it random. Or if you're not in the mood or don't have the time to actually go to a college campus, just watch lectures on Youtube.
Feed subjects into a randomizer and let that pick what you watch next. If it really is the most boring and opaque thing ever - Advanced Accounting for Financial Professionals; Principals of Marxian Agrarian Economics III; Post-modern Feminist Ontology for Golfers, whatever - then at least you can take it as an attention span challenge. Regardless of the depth of unfamiliar jargon, you'll still learn something - exactly like watching things in a foreign language you don't know, only with a lot more context because they're lecturing in English. Speculating about the meaning of acronyms can actually be somewhat enlightening in a highly technical discussion.
In fact, if there are a series of lectures on Youtube in the same course, I recommend watching them out of order so that your brain does a little bit of the work in advance before re-watching them in order to set the keystone in place. Don't strain yourself trying to "get it" at first if it's obscure - just soak and allow whatever natural thoughts occur to do so before expanding on your curiosity with further inquiries. For subjects that involve some level of work to truly understand, you can also use whatever online tutorials or other materials are available here and there.
The point is, don't just watch lectures about subjects that fascinate you or offer potentially useful skills. Be willing to gamble on the possibility that you'll bore yourself in order to discover things you neither knew about nor even suspected. And if you're too busy, ask yourself whether that's actually by necessity or whether you're just doing a bunch of shit that doesn't really serve any purpose, and you could substitute more adventurous options instead of doing whatever it is you do.