What are good projects to showcase my technical ability in a portfolio?

I'm currently employed doing some VERY light JavaScript and a lot of SQL work. My current job is moving away from the technical side of things and becoming more Project Manager/requirements gatherer. While this is the correct move for the company, and I'm really excited in the abstract for the change, I'm not interested in this - I like the technical parts of my job.

I plan on asking management in eight or ten months to move me over to the more technical side of things, after we've transitioned smoothly. However, I think they would turn me down in my current situation - I don't think they think my technical chops are up to snuff. I want to build a portfolio of projects that demonstrate extreme proficiency, something that would make me look like a real quality gem for a web company.

What should I build? What personal projects do you want to see out of someone to demonstrate technical proficiency, even if not necessarily stylish or creative?

The kind of thing I've done in the past are funny websites for my friends and I with admin functionality, a couple dozen Project Eulers in Python and Go, and a browser-based multiplayer tetris game, server written in Go. That's my current level. It's all hobby work.