It means that it's not something you do often, so you really enjoy the few times that you do do it. "My brother is never in town, so seeing him at Christmas is always a real treat"

Its a way of saying that you are really going to enjoy something. To do something that gives great pleasure. The word treat on its own has several meanings. In this context it means joy or pleasure.

It means that the skiing trips are something he or she doesn't normally get to do, and enjoys a lot.