Well it really depends on the person and the relationship you might have with her/him.
For example if I had the chance to met Sulli and she were feeling down, I would probably offer her coffee while listening to her story, reassure her on her quality, try to understand the problem, ...
It's really hard to cheer someone up in general and especially when you're not close to that person.
But you know whats always works? Jokes and funny things :) If you can crack a smile from the person, you won.
My question: If you could only watch three dramas for the rest of your life, which one would you choose?