This is probably going to be one of those "Aren't you sorry you asked?" answers, but "Green Hills of Earth" can be sung to any of the following songs, and they each and all can be sung to one another:

Amazing Grace
Mickey Mouse Club Theme Song
Stairway to Heaven
        http://www.youtube.com/watch?v=xG2lTB-UVvs
House of the Rising Sun
Yankee Doodle
Pop Goes the Weasel
Peaceful Easy Feeling
I'd Like to Teach the World to Sing
Yellow Rose of Texas
Ghost Riders of the Sky
Rocky Top
Lion Sleeps Tonight
Tangled Up in Blue
Whiter Shade of Pale
Light My Fire
O Little Town of Bethlehem
Jingle Bells
God Rest Ye Merry, Gentlemen
Greensleeves
Jolly Old St. Nicholas
The First Noel
O Tannenbaum
It Came Upon a Midnight Clear
I'll Be Home for Christmas
The Holly and the Ivy
Take Me Out to the Ball Game
In-a-Gadda-da-Vida
Marines' Hymn
Wabash Cannonball
America the Beautiful
The Internationale
Onward Christian Soldiers
Ode to Joy
Mack the Knife
A Hundred Bottles of Beer
Clementine
La Cucaracha
Semper Paratus
The Wearing of the Green
        (The Rising of the Moon)
The Itsy-Bitsy Spider
I've Been Working on the Railroad
Sympathy for the Devil
Rollin' Down to Old Maui
Acres of Clams
Bread and Roses
Sink the Bismarck
Forest Green (UK "Little Town of Bethlehem")
Tomorrow Belongs to Me
The Girl I Left Behind Me
The Wabash Cannonball
MacNamara's Band
Theme from "Leave It to Beaver"
Advance, Australia Fair
The Marseillaise (with tweaking)
Hernando's Hideaway

Poems that apply:

Jabberwocky
Stopping by Woods on a Snowy Evening
Rime of the Ancient Mariner

Little Miss Muffet
Jack and Jill
Doctor Foster went to Gloucester
Little Jack Horner
Itsy Bitsy Spider
Mary Mary Quite Contrary

Anything by Emily Dickinson


The original THE BALLAD OF GILLIGAN'S ISLAND:

Just sit right back and you'll hear a tale,
A tale of a fateful trip
That started from this tropic port
Aboard this tiny ship.

The mate was a mighty sailing man,
The skipper brave and sure.
Five passengers set sail that day
For a three hour tour, a three hour tour.

The weather started getting rough,
The tiny ship was tossed,
If not for the courage of the fearless crew
The minnow would be lost, the minnow would be lost.

The ship set ground on the shore of this uncharted desert isle
With Gilligan, The Skipper too, Here on Gilligans Isle.


So this is the tale of the castways,
They're here for a long, long time,
They'll have to make the best of things,
It's an uphill climb.

The first mate and the Skipper too,
Will do their very best,
To make the others comfortable,
In the tropic island nest.

No phone, no lights no motor cars,
Not a single luxury,
Like Robinson Crusoe,
As primative as can be.

So join us here each week my freinds,
You're sure to get a smile,
From seven stranded castways,
Here on "Gilligan's Isle."