Second, there's also nothing wrong with sometimes treating our girls to an extra-special dose of attention and indulgence. They'll probably love it. Teenage girls tend to be emotionally self-centered and physically pleasure-seeking. This is entirely developmentally appropriate, in my view: they're accruing psychic and physical reserves that they will have to draw on over the course of the terrifically demanding years of early parenting not too far in their future. The teens and early twenties are the years when young women need to be taking more than they give, storing and accumulating emotional resources. So over-the-top activities for the girls can be great—every couple of years.
If I were a YW leader, then, I would focus most on finding ways for the girls themselves to serve within the congregation. Get them invested in the spiritual work of the ward; show them that they are necessary to and valued by the institution. For young men, priesthood service functions in just this way. For young women, we might need to think a little harder, but I'm convinced that there are capacities in which they can serve the ward—preferably not always in childcare or domestic cooking-cleaning assignments. Part of the value of priesthood service for young men, it seems to me, is that it encourages them to develop the personal qualities of service and nurture that flow from traditionally female realms—food preparation, invisible personal service, social cooperation. Conversely, I think there can be tremendous spiritual growth for girls in visible leadership, management, and teaching roles within the ward.
Here are a few ideas, and I'd welcome readers' ideas in the comments. Young Women could be in charge of preparing and leading musical worship in sacrament meetings, rotating through assignments to select the hymns, display them, and direct the music from the stand. After all, D&C 25 lends scriptural legitimacy to women's stewardship over the musical elements of worship; this would be a nice complement to the men's stewardship over the sacramental elements. Perhaps girls could be given ushering responsibilities, helping to direct traffic in the chapel and seat latecomers. Maybe each week a young woman could read a scriptural text relating to the theme of the meeting at the beginning of the sermon portion of sacrament meeting.
What I like about these ideas is that they model for the girls what adult life in the church is really like: working and serving alongside men, in different and, one hopes, complementary capacities, to carry out the missions of the church. It seems to me that this is the best preparation for Relief Society we can give our girls, so much more valuable than over-the-top spiritual spoon-feeding: our confidence, our trust, and a real stake in the institution that we love.