Several apps are available for the iPad that you can use to create drawings using a capacitive stylus or your figure. A few examples are Adobe Ideas and Noterize, but those require being installed before you use them and they take up space on your iPad. If you are looking for something quick and simple consider Chalk from 37Signals.
Chalk is not an iPad app but rather a web site that is optimized for the iPad. You can access and use the free web site at chalk.37signals.com, and when you do you see a depiction of a chalkboard as shown in the screen shot below.
You can draw or write on the chalkboard using either a red or white piece of virtual chalk by first tapping the chalk piece and then drawing on the board. To erase what is on the board tap the eraser and then wipe over what you want to remove. To the left of the chalkboard is a light switch, which is mostly cosmetic but it does dim the display. In the upper right hand corner is the familar iPad Send-To button, which enables you to save the drawings that you make.