I love this app called Paper 53 from fiftythree.com
It takes no more than 2 minutes to learn to use. It makes the iPad fun. It's like having a stack of moleskin journals in your pocket. You can open them up and draw or sketch with your finger. Erase or add some color in a natural, intuitive way. It comes with some beautiful color palettes.
Now, off to make some of the paper designs into metal!