Decades Theme Party!
Depending on what you are interested in you can either assign a different decade to each grade of people that will be attending your sweet 16.
For example, the freshman might dress up as the 20's. Sophomores maybe the 60's. Juniors the 70's. And the seniors 80's.
Or else you can simply have everyone just do whatever decade they choose. It's a fun way to show your creative side. Plus how many people do you know have thought of this idea?