Flip G
Tele-Holic
I've repeatedly been told, "Just some AC/DC." 
Louisiana 1927 is a song that I have recently been obsessed with, and it is simple enough that most decent players could keep up.
This is my favorite version (warning: tasty Tele content)
I agree with Sparky if you’ve got older folks. Heartaches by the Number is another classic country song that a lot of people know and like to sing.Stuck In The Middle With You, by Stealer's Wheel. Key of D.
Jambalaya by Hank Williams. (Only two chords)
Your Cheating Heart, by Hank Williams.
I Walk The Line, by Johnny Cash.
My gig buddy Buddy sings both of those. Fun tunes.You Never Even Call Me By My NameDavid Allen Coe
Into the Mystic, Van Morrison is cool on acoustic.
Yeah, if I'm playing with more mature players the set list changes...I usually keep it to a merciful four chords max, mostly perky tunes, e.g:
Ai Ai Ay
Big Boss Man
Going Down the Road Feeling Bad
Hey, Good Lookin'
Is Anybody Going to San Antone
Little Red Rooster
Midnight Shift
No Expectations
Season of the Witch
Smokestack Lightning
You Never Can Tell
You Win Again
. . . and sometimes a fast medley: La Bamba/Sloopy/Twist and Shout/Louie Louie/Wild Thing/Angel of the Morning/Good Lovin'/La Bamba
God, no. Just shoot me now!NOT Wagon Wheel
I was at a jam party, mainly guys who were sort of yacht rock/smooth jazzers, all VERY accomplished players.Any/all of the early stuff by Tom Petty. Almost any one of them can be the original 3-1/2 minutes or as long as you like.