For the first part of my sketchbook, I only used pencil, and now I regret it. Charcoal is just some much faster and easier, if only it didn't smug so badly...
I'd try pen, but it's a little too permeant.
Side Note: I had a teacher who preached you can't get a good drawing unless it's from life. Photos would never cut it. So he's going through my sketchbook and stop at the feet picture below, and says, "This one is really good. This one must be from life, right?" It was really satisfying to tell him nope, it was a photo. He got really quiet after that.