Although this wasn't an "original" idea, going bowling has been one of our most successful yw/ym combined activities. Even the expense was little compared with the success of the event, for we had youth attend who have never attended a mutual activity before. We even had a non-member relative join us.
Another great thing about this activity was it gave the youth and the leaders a chance to "let their hair down." Sure, the boys were competitive (they still had fun), but the girls just enjoyed their time together by visiting, trying goofy ways to bowl, and laughing. LOTS OF SMILES that night. It was amazing.