One of the great things about the accordion is its expressive capabilities as far as the bellows are concerned. There's something that we do in accordion playing called swells. Basically what that means is you push your or pull to open or close the bellows and depending how hard you do that will determine what kind of sound comes out of the instrument. Here's a light swell.
A heavier swell.
Of course, this can be varied as you're playing.
Even the simplest of tunes can sound really expressive as long as you're using the bellows to push and pull and play with vigor.