Best methods to improve my programming skills?

I am a control engineer with intermediate experience and knowledge in C/C++ and Python. I want to improve my software development experience in these languages, I'm interested primarily in low-level to system level programming and a bit of GUI level using Qt.

The problem is I can't think of a project idea to stick with on my spare time out of work. We have some interesting projects in my work environment but I want to make use of my spare time in programming. Any guidance is really appreciated. Thanks!