This is a repository of plans that are either interesting, or useful but not good enough in expectation to be worth doing. I’ll add more in the comments as they come up.
I was the first (and afaict still only) person to realize, you can cure permanent lung damaged caused by COVID (or other infections) by taking 250 mg of oral Azithromycin three times a week for long periods (clinically defined as “up to two years, or until you no longer have symptoms, whichever is first”). The mechanism is the same as it is for treating Bronchiolitis Obliterans with Azithromycin. I’m not aware of any doctors who seem to know this; there are a few clinical trials using Azithromycin to treat COVID while symptoms persist. That ought to make it worse; Azithromycin opens small airways, and my understanding is that the reason airways sometimes remain closed after infection is because the body shuts them off during infection to prevent damage, and sometimes they just never open up again, so Azithromycin can be used to open them up manually if they’re closed when they shouldn’t be. So, opening the airways with Azithromycin is sort of like reducing a fever, in that doing so takes one of your body’s natural defenses against infection out of commission. Someone could get a good paper out of this if they didn’t have more important things to work on.
You can treat seasonal affective disorder (SAD) with homemade light therapy devices for around $60, writeup here.
You can reverse global warming (but not the underlying cause of rising CO2 levels) by Marine Cloud Brightening (MCB), it costs 1-2 billion USD a year to implement (plus some up-front research to optimize it, perhaps more if you need a socially legible amount of evidence that it works first, rather than a merely utilitarian amount of evidence). You could potentially go do the research yourself, and then pitch the actual project to a government, which would cost less, but I’m much more skeptical about pitching projects that are obvious utilitarian wins to governments than I used to be; people don’t care about utilitarianism. It drops global temperatures by up to 4 C, depending on how widely it’s implemented. This could potentially be very net negative, if it caused governments to pollute more, so I do not endorse it.
Olivines can be used to capture carbon at costs of around $20/ton if I remember correctly. Thanks to David Chapman for mentioning it. I haven’t looked into it in detail, but that’s better than any other climate interventions I know of.
Not my project, but good enough to be worth mentioning anyways: you can build out vans/buses to be very cheap and comfortable to live in.
Not my project either, but supplementing vitamin D is the most utilitarian anti-COVID preventative measure I know of.