For more years than I care to remember I've been a fan of carrying a stack of index cards. As a freelancer trainer I used to pride myself in being able to whip up an interactive training experience if needed short notice with nothing more than index cards and a marker. I first came across the fact that there were others like me when I saw the Hipster PDA discussion over at 43folders probably around 2007. I then had a brief peak of use with fans of the GTD productivity methods. Many people got into printing custom cards, calendar cards and more, but for me a plain old stack of cards and a binder clip was the classic. With one fault, after a few weeks of a deck of cards being in pocket they would look decidedly scruffy and dog eared. As such over the years a plethora of ideas to stop that have been deployed. The current one is this living hinge cover I drew up in inkscape and laser cut a few months back at the Ffiws Makerspace in Porthmadog. Cut from 3.125mm ply the hinge won't survive being sat on but is surprisingly robust, the clips work as well as they ever did and I use 4 clips so that some can be used as bookmarks for a current card over multiple projects. Finished with a small bit of elastic its easily pocketable and a good conversation piece when people notice the hinge section. :)
If you have access to a laser during these times (lucky you!) then I've posted the file on thingiverse for anyone to use.