I use some cards with ideas and general concepts on them, they are good for little 'on desk' presentations to clarify stuff while in meetings and conversations. I got that idea years ago on this forum, btw.
Because of that I also have my @agendas on index cards. I use the free ones without lines or some such.
I also have some empty cards with me for doing capture. The rest of my system is digital. Sometimes I use a paper notebook for capture, but it's more for casual one-off occasions.