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 always have a variety of pens and pencils with me, and writing/drawing with high-quality pens & pencils on real paper is better and more convenient in every way than writing on an equivalent-sized whiteboard surface. I then have 100 pages I can keep or give away instead of one that I have to delete and reuse, and without all the mess. Plus, I always have a couple of pens in my pocket, even when I don't have my laptop, so I never end up with a whiteboard but no markers.

The only time I've found whiteboards more useful than paper are when I needed a very large surface. If a small surface is good enough, paper works better for me.