This project has an industrial design feel to it, which I love, and it’s really very simple to carry out.
Concrete lightbulbs could be used for a few different things including wall hooks and decorations.
Go here for the instructions.
P.S. Some lightbulbs contain mercury, so it’s suggested (in the comments) that you work outside with a mask on. Loads of people have done this project without a mask, but better safe than sorry!