I think everyone has their own program or set of features beyond "Hello World!", that they use when trying out a new language.
Mine is a guessing game:
I'm thinking of a number 1-10, guess what it is! Guess: 3 Nope, too low! Guess: 7 Nope, too high! Guess: 5 Yes, You win! Play again (Y/N)? N
What do you write?