This is a tricky problem in general, and specific, partial solutions abound. Especially unidirectional solutions. For example, you can get read-only versions of wikipedia for offline use in your remote mountain village; but there is no easy way to contribute your updates back to the version on the main internet.
Also you should have the internet cached for offline use even if the net is nice right now, because nation states are war gaming to destroy the internet, and us little people will suffer when that happens and we can’t get our Youtube instructional videos on how to survive the apocalypse after it happens.
Offline automatic filesync
See mitigating link rot.
AFAICT there is no way to contribute upstream. But a reasonably simple and well-curated option is to use the Kiwix offline wikipedia, which can give you everything, everything minus pictures, or only “medical” articles, or only “school” articles and so on.
devdocs.io is an excellent offline cache of API docs that works from your browser.
I cannot tell if Fallback is a really viable project, or an art project designed to make a point. Filip Visnjic’s review gives an overview: caching internet news just as a nation imposes internet shutdown on a restive population by monitoring net activity
Fallback is triggered by powerful forecasting algorithms providing a backup right when it is needed. “We constantly monitor the probability of Internet shutdowns worldwide” — Quifan tells CAN. The prediction is done by trend analysis of the appearance frequency of certain keywords in the online world.
[…]the system scrapes headlines and articles from news platforms, encrypt it, and send it over satellite to the Portal devices (Raspberry Pi Zero W with E-ink Module). Portal receives data over satellite, decrypts it, formats it into news articles, and provides its own WiFi access point where no Internet is required.
Offline file sharing
PirateBox creates offline wireless networks designed for anonymous file sharing, chatting, message boarding, and media streaming. You can think of it as your very own portable offline Internet in a box! PirateBox project is following a view simple goals.
It lists certain peer projects
Yes, check out Aram Bartholl’s fantastic Dead Drops. There are also several forks of the project, including Jason Griffey’s LibraryBox (actually nearly 1:1 and working togehter)., the Bibliobox and the CoWBox (CoWorking Box). A new C.H.I.P based PirateBox fork is AnyfestoCHIP (Project-website). In addition, there are several projects which focus education, creating an offline network, but are not based on Piratebox: