There's a great little session tune that I got from John Kirkpatrick's 'English Choice' book. It's called Bobby Shafto (no, not *that* one).

Every time I play it I can guarantee that somebody will say 'that's a great tune, what's it called?'. It's very easy to pick up too so, by the time you're on the third time through, everybody has joined in!

If you don't want to buy the book (or dots are not for you!) it is on John's 'Orlando's Return' CD.