I also like the idea of Sea Shanties, but if you don't know one it's hard to start them off on it. The Roll the Old Chariots is a great one as it is a very easy one to pick up. Another song that is almost more universally liked along with This Land is Your Land, would be an old spiritual, Will the Circle Be Unbroken. It's something that I'm sure a goodly portion of any audience in North America would be familiar with.