It is the mindset you have about that relationship. It is not the type of relationship one is in. Having a "weekender" relationship is exciting. Anticipation builds between each romantic rendezvous. There is a rush of feelings and thoughts about seeing each other and imagining what you will well... do. When one is in a steady relationship the mystery of what will happen diminishes because of habit.
The key is to build this excitement in the steady relationship, Pack your bags, grab your girl, and head off for a mystery weekend. Don't forget to pack a special little something you fantasize seeing her in. However, make sure it is something that she will feel comfortable wearing.