Comment by boricj
If you're looking for suggestions outside of work, one important factor will be motivation. You're not paid for it so you'll need something that will hold your attention regardless, or you won't commit to it. Therefore, the answer depends on what you personally find interesting.
Yea I thought about that, probably game dev from the ground (with my own graphics engine) up or a web app that I find useful (currently no thoughts on that).