Common ones from when I started writing video games in C in the 90's - * Write malloc() and free(). * Write a gzip decompressor. * Write a triangle rasterize

Common ones from when I started writing video games in C in the 90's - * Write malloc() and free(). * Write a gzip decompressor. * Write a triangle rasterizer and use it to draw a spinning cube on the screen. Each of those is a decent but manageable amount of work for a new dev and will teach you a variety of useful low-level skills.