For the creative part of you interested in drawing out an idea, scene or picture, consider Paper by FiftyThree.
This uniquely intuitive app allows you to capture and share ideas using various tools and colors.
With several books suggesting that doodling and drawing out your ideas are good ways to improve your persuasive and thinking skills, this app is a good place to start.
Of course, you can also by a “Pencil” to allow you to interact with the app in some fun ways like smudging and erasing. The pencil is not required but feels natural to use.
If you are interested in some books on drawing out ideas, check out:
The Back of the Napkin:Solving Problems and Selling Ideas with Pictures.
Drawing Ideas: A hand-drawin approach to better design
The Doodle Revolution: Unlock the Power to Think Differently