Earlier I use to implement tower of hanoi and later client/server app, which (in my mind) would convince me that I knew the language to some degree. Obviously, every language has some unique feature which need not get exercised by these generic apps, still what is your go to application which boosts your morale with new programming language, once you implement it yourself?
<a href="https://www.programmingsimplified.com/c-program-generate-fibonacci-series" rel="nofollow">https://www.programmingsimplified.com/c-program-generate-fib...</a> in combination with writing tests for the output.