I constantly have my iPad in my hands when I'm relaxing and not doing much of anything. One of my favorite apps is Paper by 53. There are much more advanced drawing programs for iOS, but this one has won me over, probably because it's so minimal.
There's a lot of things that you can't do in this program. You can't zoom in to add detail, so you're limited to what your fat little fingers will allow you to do. You can't clone, blur, change opacity, draw on different layers or a million other things that I've grown accustom to in programs like Photoshop.
You have five brushes, an eraser, undo, and a color palette. That's it. The brushes are really well done, it's hard to describe, but they have a really nice feel to them. The color mixer is also really well done, there's a very detailed article about how they went about designing it. It's very impressive to see the amount of research that was done for something that you would think would be a basic feature.
Because you can't zoom in, you can't get bogged down into the details of your images. You have to work with the big picture, so I tend not to fixate on things. I just 'let it flow', and while none of my pictures turn out to be perfect, I find I don't obsess over the little details that often stop me. I've done dozens of sketches in this program, while the technically more advanced Sketchbook Pro doesn't get much use from me.
Here are some of my fun sketches that I've done, usually laying on the couch half-listening to the TV.