Content packages they have available for download (and which can be viewed online with them): https://library.kiwix.org/
Being able to download all of Gutenberg's public domain books or Wikipedia are the classic Kiwix choices in my view
Partially related article: https://www.theverge.com/2022/10/20/23413615/stack-overflow-offline-coding-copy-kiwix-prison-arctic
saw some new content was available for download since I posted on this last year, like openstreetmap's wiki, ifixit stuff, lowtechmagazine, etc.