I, actually, helped out a girl with whom I go to school who needed a Sondheim song for a class. Most of the most popular ones had already been taken. (She ended up doing "The Girls of Summer".)
So, I would recommend some of the lesser known Sondheim songs:
"Class" from "Saturday Night"
"With So Little To Be Sure Of" from "Anyone Can Whistle" (Check the vocal selections first. I think they have it as a solo.)
"Water Under the Bridge" from the never done William Goldman/Rob Reiner film, the name of which escapes me.
"Multitudes of Amys" cut from "Company"
"A coherent existance after so many years of muddle" - Desiree' Armfelt, A Little Night Music
"Life keeps happening everyday, Say Yes" - 70, Girls, 70
"Life is what you do while you're waiting to die" - Zorba