Depends on your style of learning, but most people learn best by implementing something in a new language. Here are a few examples: a Brainfuck interpreter, a raytracer, even command line tools like ls, cat, grep are good exercises. If you are into electronics C is great for embedded work - I would buy a Raspberry Pi Pico and write C programs for it.

I would also suggest to enable all warnings your compiler allow for and sanitizers if possible. You will also want to learn to use a debugger that works on your operating system.