For me it usually stops when I mentally calculate how much work it requires, and I realize I’d rather just play video games.

I came up with idea where instead of typing stuff like “5/6” “6*9” into the terminal, you could have gui interface.

Yeah don’t let this stop you! If you do the side project for fun and/or learning, just go ahead and build stuff. Don’t look at other projects too soon so you give space to your own creativity. But perhaps compare stuff in a later stage.