Have the students close their eyes as you play the second movement for them. Others will need some coaching to hear it. We call that a theme and variations, because each time it is played, it is varied a little bit to make it sound a little different. As they listen once or twice and you point out at the blackboard each of the four sections as it is playing, have them give you their feedback. The four sections could be described in a number of ways.
Soft strings play the melody the first time.
You could ask the class why it doesn't come back and point out that it wouldn't be a surprise anymore, since we'd be expecting it in the same place in the melody after we've heard it once. The melody plays again with violins doodling and noodling around as a kind of elaborate decoration to the melody. The full orchestra plays, brass and percussion families included. Even though it is not so easy to hear individual brass and percussion instruments, it is clear that the sound is much fuller and louder. You may be surprised at how simple that melody is and what a wonderful piece Haydn made out of it.