Every time someone mentions an interesting book, I make a note of it. Then a couple times a month, I go back through my notes, pop open a website that shall not be named, snag em all, throw them into calibre-web and sync them down to my ereader. I can get hours of enjoyment from a 1-2MB file, and I love that. Same for older cartridge-format game roms, a N64 rom is generally under 50MB and can keep me busy for days, an NES or GB ROM is usually smaller file size than a book, and some great old DOS games clock in at a handful of MB.
What other great bang-for-the-storage-buck stuff is out there that you enjoy?
for music you can get into .MOD files, theres millions online and you can edit them and make your own also. you can also emulate computers like C64 and Amiga for endless fun.