[ylg]

I find my interest is most piqued by non-programming fields. So, for me the answer to this question has been: listen for opportunities in the communities around fun, non-programming fields. In my case, listening to a NASA Twitter account led to a weekend hackathon that showed the possibility of programming around live telemetry from space vehicles, which I've been noodling with ever since: https://github.com/sensedata/space-telemetry

Another idea I've been messing around with for ages is a mobile app to support bicycle wheel building; something else seemingly non-programming that I love doing and so looked at specifically to see if I could invent a way to a hobby project.

And, games are another arena that have worked well for me, i.e., programming around them is fun enough to keep me well enough engaged to learn and understand new things. Mods, play-support tools, and of course actual (mini) games. If this sounds interesting, I recommend finding a moddable indie game to start with; micro-studios can be very friendly and supportive of modders in ways that keep you coming back for more.

PS From the OP's context, I'm assuming the real question is "How do you come up with hobby, programming projects that keep your professional abilities fresh and varied?"