Working in pairs, exchange data between Processing and Arduino using serial communication. Make a Processing sketch that tells Arduino to do something, and make your Arduino tell Processing to do something as well. Document your work on the documentation blog.
One way to think about this is to send a char or a byte from Processing to the arduino but make sure you know which one you are sending. Also make sure you have the serial port number set correctly and the baud settings set on both Arduino and Processing.