Never heard of "happy as a sandboy." Never heard of "sandboy." OK, I just found out that a sandboy is a boy who sold sand, which I assume is an obsolete occupation. But why should a sandboy be happier than most?

We have a similar expression "happy as a clam" which always seemed rather surreal to me. Why should anyone think a clam is happy? But I just discovered that the original expression was "happy as a clam at high tide." Now that makes sense.