What are one or two instances of a metaphor that is used in "The Road Not Taken"?

To my way of thinking, the whole poem is metaphorical.

If you take the poem literally, there is very little metaphor in the poem.  The only place where you can really argue that metaphor is being used in this line:

Yet knowing how way leads on to way
I doubted if I should ever come back.

This is metaphorical because he's not really saying that he's going to keep following various roads and be unable to ever return to the spot where he stands.

But really, the whole poem is a metaphor.  The idea of forks in roads is just a metaphor for decisions that have to be made in life.  They are like forks in the road in that we have to choose which direction to go.