Go for something with an easy chorus picked up second time through or something they will know.
Dirty old town
Drunken sailor
When the boat comes home

or irish pub songs.
For the 20 somethings these are new and refreshing - often the first time they will have heard live accoustic music outside of school.

I play Biker hill in em and intersperse it with go and enlist as a tune - trouble is the words get a bit lost when you play it at a good dance speed.

Enjoy your do.
FloraG.