We had to design and build a simple robot on a recent university course I attended. I chose to build a drummer robot that listens to a rhythm, and then constantly taps it with its little hands. It also plays an improvised melody with a simple algorithm that just moves up and down a major scale.
It’s running the LeJOS operating system, which runs Java. It works surprisingly well considering the NXT-brick has only 64 kB of RAM :)