There are plenty of fun things you can do with a decent 3D printer and Arduino. Samy Kamkar’s lock cracker is pretty interesting as well. The Combo Breaker is a motorized, battery-powered Arduino-based device that can crack any master combination lock in a flash. He used an Arduino Nano micro-controller, a stepper motor, a 500 mAh 3S Lipo battery, and a few other parts to pull this off.
The good news is Samy has made the code for this project available on GitHub. In the above video, you will learn more about the Combo Breaker, and how you can build your own.