Start with something like Kiwix. You can use that to download ALL of wikipedia. You can keep that on a thumb drive.

After that, start thinking hard about what you might want to access.

I like GOG for gaming, because I can download the DRM-free installer without the need for the internet later.

Whatever eBook library you “own” right now? Download it.

Do you still have a bunch of DVDs? Preserve them.

Any CDs sitting around? Rip those tracks.