It is no secret that you can build all kinds of fun things with a decent 3D printer. This is a 3D printed solar Stirling engine that you can build without needing a milling machine or a lathe. It can be powered directly by sunlight. When the weather conditions are not favorable, you can rely on reflector mirrors to increase the efficiency. The engine even works indoors.
The above video shows this 3D Printed Solar Stirling Engine in action. It measures 270 mm x 330mm x 370 mm. The good news is you can buy the construction manual to build one of these for yourself.