I grabbed the free version of the SketchBookX app from the app store the other day. I gotta say, this is one of the best apps a creative person can own. I've been using it whenever I get a seat on the train and/or bus on the commute to and from work (you need two hands once you get into it). The free version gets you 3 layers to play with, a pen & brush tool, an eraser (wish this had a brush size option), color wheel, and a tool that creates a symmetrical version of every brush stroke you make. Here's a sketch I did on my way down to Lucy Parson's Center and back home (2 orange line trains, 2 red line trains, and a bus ride).