There are a variety of ways to use keys as an idea for poetry.
A poet could focus on the shape of the keys and use that as a metaphor for something or use it as a comparison point.
A poet could examine the idea of the metal in keys and the different attributes that it may have.
As an obvious idea, a poet could look at opening/unlocking and closing/locking. A poet could examine various situations in which a key opens/unlocks something or closes/locks something. A poet could examine different types of things that are opened and closed, such as doors, boxes and lockers.
In addition to metal keys, a poet could look at electronic ones.
Additionally, keys, like most other objects, could be looked at from a metaphorical point of view.