The only thing that really got me going was small applications I had some interest in. Writing games I will not play never kept my interest up for long. So I’ve been building mini tools I used for teaching numerics classes in meteorology (Python, Julia, Fortran, C) or code I would be using for some tinkering with microcontrollers or similar (Python, C++) when using new languages. For me personally, some iconic projects were a CO2 sensor with an attached display, or very simple internal gravity wave ray tracers. But that’s likely not what you’d be interested in. So without trying to suggest specific applications for you (many good examples in the responses :-) I’d advise to do something you’ll have fun with. Get yourself a small project that generates added value for you specifically (and fun is great added value in my eyes).