I will be the gladdest thing
Under the sun!
I will touch a hundred flowers
And not pick one.
I will look at cliffs and clouds
With quiet eyes,
Watch the wind bow down the grass,
And the grass rise.
And when lights begin to show
Up from the town,
I will mark which must be mine,
Millay’s soothing interactions and observations of nature also have a rebellious nature in them–traditionally, grass bows/sways when wind pushes them. 
 This may be a metaphor she is relating to–a metaphor for rising up against repression, or being brave enough to be unique.
Afternoon on a Hill