The masters of the Viennese Classical school (Haydn, Mozart, Beethoven, Schubert) often have a quality to their music which has always spoken deeply to me. It's an unfolding of melody, a skeletal structure of progressing pitch and rhythm produced by developing a simple theme or two, that carries us along on a seemingly effortless river of thought-in-sound.
Of course we know from Beethoven's extensive sketches and from Mozart's letters that these works were often produced with great effort. Here is one simple and enjoyable exercise that can help us to appreciate this.
Play a recording of a famous symphony (I have some suggestions below) and then hit pause somewhere, perhaps after a phrase seems to complete. Then try to guess what the music is going to do next by singing in your mind an imaginary phrase, an "aural-ization" of the direction the music might go. Often, unless we try really hard, we simply imagine the phrase we already heard being repeated.
Then hit play. The music may repeat for a moment, or not, but it's off in a direction that makes us think "Of course that's what it does! What else could it have possible done? What didn't I think of that?"
Great examples of this are the expositions of the first movements of Mozart's Symphony #40 in G minor or Beethoven's Symphony #5 in C minor. Or the opening movements of Mozart's "Dissonance" Quartet (No. 19) or Haydn's "Fifths" Quartet (Op. 76 No. 2).
Or Haydn's last symphony - #104 in D Major nicknamed the "London" Symphony.
I absolutely love this quote from "The Enjoyment of Music" by Joseph Machlis with Kristine Forney, speaking of the first movement of this piece.
"The movement proper, an Allegro in sonata form, is launched by an irresistible tune that has all the energy and verve of Haydn's mature style. This theme and those that follow are shaped out of a few basic intervals in such a way that, despite the variety of the ideas, they all hang together. There results the effortless continuity, the miraculous sense of rightness and inevitability that constitutes the essence of the high Classical style."