I "upgraded" the internal storage of my Kobo Glo HD (which is in fact using a 4GB MicroSD card as its onboard storage) to a 128GB. I can store the "all maxi" 82GB ZIM file on it. The Kobo firmware includes (but doesn't advertise) ZIM support, so I can browse Wikipedia on an eInk display and great battery life without Internet connectivity.

That would be a great post-apocalyptic knowledge archive, and it would be easy enough to recharge using a USB solar panel.