One thing that's always good at sing-alongs is a song that's call-and-response and/or that has an easy-to-grasph, repetitive chorus or refrain.   Songs like The Wild Rover or Deep In the Heart Of Texas, with clapping parts, also work well for audience participation.

One reason I often do La Bamba - even for audiences that don't know a word of Spanish - is that all they have to do is sing "Bamba, bamba, ... etc." and then make all kinds of festive noices. ; D