The two most obvious things being contrasted in "Because I could not stop for Death—" by Emily Dickinson are life and death.
The first difference between the two as portrayed in the poem is that life is only temporary but Death marks a transition to eternity and immortality. Life is the realm of change, from infancy, through childhood, to adulthood and old age, but death is timeless and ageless. Seconds and centuries are all alike in the realm of Death.
Life is the realm of the body, and of physical realities. This might include such everyday things as scenery, houses, schools, children, food, and clothing. In death, one escapes from the physical to the spiritual realm. In a sense, Death is portrayed as a transition from the realm of the human to that of the divine.