Hate to be stereotypical, but the classic Chopin Funeral March would be beautiful. I always loved it because it was so somber and honest, but also has an amazing and beautiful melancholy section that is incredibly peaceful.
That or Gymnopedies No. 1, which is just a peaceful classical piece that's a favorite of mine.
If we're going into modern and random songs, I want a piano cover of "Don't Give Up" from Undertale, which is just incredible.