Post-pandemic: TRAVEL. Nothing in the school curriculum can teach what travel can. Books only do so much. If unable, game with people from other countries and talk to them. Talk to people from other countries. Strike up a conversation and listen. Use the internet and international video gaming to your advantage. Books are so full of assumptions of other cultures, so many mistakes, and you don't get the truth unless you've actually been there or know someone who is there. There are a couple ways of traveling for free or low cost included but not limited to: find jobs that seek Americans in the country you're curious about, find a nonprofit that sends you over somewhere for a good reason, or join the Peace Corps. If you live your whole life without ever leaving the country, you have no idea what you're missing. Seriously. Also note: if your job requires you to be a desk jockey and you only need an internet connection to get to work, who says you have to stay in one place anymore? The digital nomad lifestyle is not just for perfect-life-sellers with an Instagram account anymore. Something like 40% of the US population is now working from home. If you can work with just an internet connection, then you can work from anywhere in the world that has one.
Guide to cheap travel abroad Nomadic Matt, Migrationology and The Broke Backpacker
CIA World Factbook and ThoughtCo Articles for Geography
I highly recommend that if you are unfamiliar with war, you ask a veteran friend if they know anyone who would be willing to talk to you about it. Civilians don't know anything about it. Not a damn thing. We can only get closer to the truth, and the only way that's gonna happen is by hearing it from people who have been there.
Social Ideas and Issues
CGP Grey Miscellaneous Social Studies topics
Voting in the United States
Illuminaughtii Youtube Channel
Steak-Umm's Twitter Megathread Also contains basic critical thinking info
The Narrativist Framework
The Evolution of Trust: intro to game theory
The Crowd by Gustave LeBon A book on mob mentality. Sexist, racist, flawed, and heavily reliant on assumption, it is very much a product of its time. It is still worth reading.
Implicit Bias Test These include bias tests for racism, ableism, and other things. Provided by Harvard University. Test yourself, see what you get. Take it with a grain of salt though.
TV Tropes especially Old Tropes
Big Brain Time