I'm not being glib here, but after experimenting with many such solutions over the years, I've found that the best solution for me is to always carry a cheap, paper notebook--like one of those black/white-spotted composition notebooks with cardboard covers and unlined paper--that is about the size of my laptop lid. I just slide the laptop and notebook together into whatever backpack or bag I'm using for the laptop. I…
On a different note, notebooks are great. I converted my wallet into thin Moleskine notebooks that fit in my pocket super easily. I put my cards and bills in it. And I always have a pen in me. Archival pens are great. It's good for doodling too :>