Cargo-Bot during Hour of Code and found it to be a great game to teach programming concepts such as loops and conditional logic. It works a lot like Lightbot.
The only drawback is that it does not support multiple users on a single iPad. This would not be a problem in a 1:1 environment but I have a single device to use with multiple classes.
For those who want a bigger challenge, the entire game was built on the iPad using Codea. Not known for its programmability, Codea unlocks the iPad's potential for budding developers.
Cargo-Bot was a free app, while Codea is paid (which I have not purchased).