Simple:
* The blues musician blew a mournful melody on his harmonica.
* My grandfather used to play the harmonica in a band.
* She bought a new harmonica for her birthday.
More descriptive:
* The harmonica wailed like a lonely wolf, echoing across the empty field.
* With a few deft puffs and draws, the musician coaxed a sweet melody from the harmonica.
* The harmonica, a small but powerful instrument, could convey a wide range of emotions.
In a story:
* As the train rattled through the countryside, the old man pulled out his harmonica and began to play a tune that was both familiar and comforting.
* The harmonica was more than just an instrument to her; it was a connection to her past and a way to express her feelings.
You can also use "harmonica" in a more general sense:
* The band included guitar, drums, and harmonica.
* Learning to play the harmonica is a great way to develop your musical skills.