People have programmed PR2s to do all sorts of fun things, but most of these things are human-y fun, not robot fun. Georgia Tech has built what they're calling a "PR2 Playpen," which is designed to keep their PR2 busy (and entertained?) while teaching it valuable life skills at the same time.
Robots tend to have difficulty when it comes to interacting with unfamiliar objects. We've seen some cleverways to at least partially circumvent this, but to perform best, robots need to be taught how to pick things up and then practice with different algorithms to find what works. This is rather time consuming, and the PR2 Playpen is designed to help automate the task. A conveyor belt drops random objects one at a time into a workspace in front of a PR2, which studies them and attempts to pick them up. The robot can autonomously evaluate different controllers and strategies, while collecting data to improve its algorithms at the same time.
The other benefit to this system is that the robot can keep itself busy and productive without any human supervision. There's a limited amount of time that research labs can keep their PR2s hard at work, and I bet that even the busiest lab would have trouble actively using their robots more than 50 percent of the time, if that. With the PR2 Playpen, the robot can spend all night every night teaching itself to get better at grasping things, and thanks to ROS, all robots everywhere will be able to benefit from what it learns.