So, we've had rate the above persons avatar and ban the above person, but those threads can bring people down. This thread is intended to bring people up! Make someone happy! Do what you want to make the above person happy!
So for example, you could say "I like the above persons hair" or "I love the way you are so nice", or even just a friendly gesture like a hug.
But, just saying "good job by the way" or others similar to that will not cut it. So don't say it.