When I was in school I would often doodle in class, just on a plain sheet of paper with a ballpoint pen. There's something therapeutic about not being able to fix mistakes and to draw my brains out. At least 90% of the drawings were crap, but there are always a few that turn out better than I expect it.
I did have an off day today, just could not get into the grove of things no matter how hard I pushed. I reminded myself that the first rule of art is to relax and have fun. So I picked up a sheet of paper, a ballpoint pen and my neglected brush pen, then just went at it for the next hour to explore some ideas for an upcoming painting.