"Stopping by Woods on Snowy Evening" is one poem that could represents life's journey. With "miles to go before I sleep" as a representation of the long journey of life, this poem is symbolic in that life is a long road filled with "promises to keep." Although there is not one overt symbol, there is a journey which can represent life's journey. Also, dark woods is a powerful recurring symbol in Frost's poetry.

Virtually any good poem from a good poet should be full of symbolism as it is one the most widely used literary tools. However, here are some of my favorites. Fire and Ice by Robert Frost creates clear and strong symbolism with clear imagery. I Know Why the Caged Bird Sings by Maya Angelou also creates strong imagery and uses the perspective of the bird and his cage to symbolize a number of important themes.

Another Frost poem that has meaningful symbolism is "Birches." He pursues several overlapped ideas in the poem, then rounds out their relation to his theme with the  symbolism likening the troubles of life to a trek through "a pathless wood." At the end of the trek, he'd like a break from it all for a while by going back to being a bender of trees like the boy on the farm, whom he described:

It's when I'm weary of considerations,
And life is too much like a pathless wood
Where your face burns and tickles with the cobwebs
Broken across it, and one eye is weeping
>From a twig's having lashed across it open.
I'd like to get away from earth awhile
And then come back to it and begin over.

Symbolism is when the poet names/describes an actual thing that also has connotations associated with the thing.  Sonnet 73 by William Shakespeare describes three actual things that die out in this sonnet.  The point of the poem is a discussion of his own aging.

"The Darkling Thrush" by Thomas Hardy is a poem about a bird, but the bird is a symbol of hope which the poet specifically discusses in the final stanza.

"Blackberrying" by Sylvia Plath is a poem about the act of going blackberry picking, but the act is all a symbol for difficult and unfulfilled journeys in life.

"The Silken Tent" by Robert Frost is about a tent, but the tent could be a symbol of a women and/or a symbol of his poetry.

I agree with post 4 and would only add that most poetry by Robert Frost is riddled with symbolism that isn't terribly difficult to analyze.  As a teacher it seems symbolism is an intimidating subject for most students (because it is so abstract), but Frost's poetry tends to blend a unique balance of the concrete and abstract.  You could almost pick any of his poems that you enjoy and find symbolic elements within.

Another poem by Frost, "The Road Less Traveled," might be one to consider. This poem talks about choosing between two roads, and it is frequently misinterpreted. Many believe the roads are symbolic of life, and that we should have the courage to choose the road less traveled and make our own footsteps in unchartered territory. However, there is, as indicated in the poem, very little difference in the amount of travel the roads get. We simply should make our best choice and deal with the rewards and consequences.

A poem that I like that has symbolism in it is "Mending Wall," by Robert Frost.  In that poem, the wall that the neighbors are mending can be seen as a symbol of the wall that separates individuals from one another.  The neighbors' conversation can be seen as a discussion of the extent to which it is important to maintain a distance between one's self and others.

When introducing Poetry, I often start with Dr. William Carlos Williams's "The Red Wheelbarow" and "This is just to say."  I think you will find both poems useful in your task as they are both accessable and replete with symbolism.  "The Red Wheelbarow" is a seemingly simple portrait with words of a wheelbarow outside by the chickens.  A cursory look at this poem will send many readers away thinking it is a simple poem - but look closer; write the poem out horizontally, scan it (I think you will find it very close to iambic pentameter), then think about what it says.  What depends on the wheelbarow? Where is the speaker?  I would suggest looking at possible symbolism within the poem the red wheelbarow, the rainwater, the white chickens, but then at the symbol that the entire poem creates.  This poem fits neatly into a category of poetry called "imagism" and highlights one of Dr. Williams's famous poetic concepts: "No ideas but in things."