We had quite the crew today with 10 Activity Day girls! I started out reading from the Strength of Youth pamphlet regarding dress and appearance:
Servants of God have always counseled his children to dress modestly to show respect for him and for themselves. Because the way you dress sends messages about yourself to others and often influences the way you and others act, you should dress in such a way as to bring out the best in yourself and those around you. However, if you wear an immodest bathing suit because it's "the style," it sends a message that you are using your body to get attention and approval, and that modesty is not important.
Immodest clothing includes short shorts, tight pants, and other revealing attire. Young women should refrain from wearing off-the-shoulder, low-cut, or revealing clothes. Young men should similarly maintain modesty in their dress. All should avoid tight fitting or revealing clothes and extremes in clothing and appearance.
As Latter-day Saint youth, you can also show respect for the Lord and yourselves by dressing appropriately for Church meetings and activities, whether on Sunday or during the week. If you are not sure what's appropriate, ask for guidelines from your parents, advisers, and bishop.
I then showed the girls pictures of women wearing immodest clothing and asked them what was wrong with what these women wore. They quickly caught on with things like: her shoulders are naked or her skirt is too short. I think it helped to see the models instead of just talking about the clothing.
Next we had the girls pair up, this was done with assistance as two girls both wanted to be partners with the same girl. They traced out one of them on newsprint and were told to create a modest outfit using scrapbook paper, crayons, glue, glitter, yarn, pipe cleaners, etc. They worked together in these little teams which was great. Everyone seemed to get their creative juices flowing. It was a good activity which kept them happy while learning at the same time.
This idea came from http://ldsactivitydayideas.blogspot.com/2009/09/modesty.html