I'm constantly commended on my side projects, and I excel at them because I enjoy them:
- https://www.youtube.com/watch?v=x034jVB1avs
- https://www.youtube.com/watch?v=5XTi-jf-ans
The laser stuff made me wealthy, and that wasn't even my intention. People at the time told me it was a huge distraction from my academic career and a waste of time.
My current side project is a real time voice converter (like Mission Impossible).
Follow your passion. You'll deliver better results and open lots of doors.
Your side projects will also usually be pretty novel. It's fertile ground for startup material.