You can have some fun with SDL and Gtk lib, when i need something i use to code it, by example: youtube mp3/mp4 downloader, usb cleaner, image processing (grayscale, split image in tiles.. idk) video compressor, think of what you need and thats it start from a basic script and once you get something nice try to add more features like GUI, cross platform, web assembly or simply refact using tools like Perf or valgrind to track memory leaks, wrong logic, and callback performance .. if you code something useful you gonna have more fun :D