Adobe Sketch is an simple sketching app for the iPad. It's designed for use with Adobe's Ink & Slide, but works almost as well as a stand-alone app, and is free to use (you'll only need an Adobe ID).
A clean and simple interface gives quick access to a few basic drawing tools - pen, pencil, marker, an eraser and so on - and various colour picking options (choose a colour theme, select a custom colour).
Sketch is a little smarter than it looks. Brush size and opacity respond to your drawing speed and pressure for realistic results; there are some helpful new multi-touch gestures (swipe two fingers left to undo, right to redo, three fingers to browse history, two finger tap and hold to pan); and Creative Cloud integration means you're able to save sketches to your storage space as required. (You only get 2GB in the free offering now, but that should keep you going for a while.)
You don't get any real editing tools, though, or layers, or anything even faintly advanced. Sketch is all about reproducing the feel of natural drawing, and so fine control is limited, and if you make a mistake you'll probably just have to undo it and try again,
Sketch is seriously short of features, but then that's why it's free. Still, what you do get looks good, has a clean and simple interface and is capable of producing great results.