Something you can teach others! Here are some ideas:
- Why do the <br> and <hr> elements (or any element) exist?
- How to create a slideshow with the least amount of code?
- What is the DOM? Why does it exist?
- Check how browsers implement a specific property and compare it to the spec.
- How a popular website implements their homepage (with suggestions for improvement)
- How you would fix a popular website's faults (long load-time, invalid JS, etc...)
- Take a popular demo and make it something else (or use it practically).
- Make a summary of popular ideas around the web lately
- Your experience contributing to an open source project.