A basic example that has many features is a temperature converter, basically fahrenheit to celsius. The temperature calculator is for programming langueages the second Hello World! Why ? because we have text input, text output and processing.
For object oriented programming we can have classes to handle the temperature value, where the constructor can auto convert to all available scales.
I researched the web and found a list with temperature converter in 7 programming languages.
You can find the list here:
7 ways to build a temperature converter