Not-hymns at wedding

One of the things that's traditional at weddings we'd like to include if we can, is something everyone sings. Does anyone have any suggests for _what_?

Ideally it would be:

- a tune many people know, at least a little bit, and we can get enthusiastic about singing
- not so Christian (or Jewish) we feel uncomfortable (I'm pretty relaxed about this, but some hymns will be a bit much)
- at least tangentially appropriate to the wedding, or at least, not actively opposed to romance

Does anyone have any obvious suggestions or good ideas? Or bad ideas, for that matter, they may still inspire us with some other idea :)