Not everyone is doing the job they want. Some people are in a job because it pays the bills and they are decent enough at it. Some people are in their current job because they thought it was what they wanted to do, but turned out it wasn't.
So, what job would you do for free? Maybe it is a current hobby that could grow. Maybe it is part of your current job, but you don't want to deal with the other stuff.*
*As an example, my buddies who are flying aircraft say they fly for free, they get paid to put up with all the other BS.
For me, currently it would be design and build manned space system architectures. Figuring out how humanity can go to the Moon, Mars, & beyond in a sustainable fashion. It occupies my mind's idle time and leads me to building spreadsheets to do comparisions between existing capabilities, drawing sketches of my thoughts, and even the occasional bit of mission analysis. I already do it for free; I just haven't found a good way to get my ideas to those who can critic/use them.
What would you do for free? (or maybe just enough to make ends meet)