you could make them custom tank tops. get white tanks from old Navy or target or even Walmart and print your design of photo transfer paper. Maybe have the front say something like "Kesley's Sweet Sixteen Posse" or whatever , and then have each girl'a name on the back of her shirt?
Or bring Glow necklaces and bracelets to give them at night. I would have loved that at 16 (heck, I love that at 31)
A Disney photo frame to put their Splash mountain photo in.
Bring some supplies to let them decorate their magic bands (Google how to do- most people do temporary tattoos, nail polish, glitter polish, fake jewels, etc)
Along the lines of a t-shirt, buy some cheap canvas bags in her favorite color and make custom trip bags for the girls. to use.