Based on the provided text, Lucy plays the bass instrument only. She does not play the flute or the trumpet, as those are already assigned to Mary and Pete respectively. Lucy is also mentioned as not playing a wind instrument, which in this context would rule out the flute and the trumpet since they are both wind instruments. The only instruments left unassigned and not falling into the category of wind instruments are the drums (already assigned to John) and the bass. Therefore, Lucy plays the bass instrument.