Pythonista is a groovy app for iPad that lets you do programming in Python right on a device without needing access to a “full” computer or keyboard (although a bluetooth keyboard certainly helps!). It has some pretty good libraries for working with graphics and the input capabilities of the iPad, too.
I was playing around with it while trying to make something to demonstrate to students interested in programming, and came up with this little gravity sim. It was a neat demo of what can be done with just a tiny bit of code.