Hello Margaret! A sweet 16 at Walt Disney World sounds like a wonderful way to celebrate such a momentous birthday. How fun!
The first thing that popped into my mind after reading your question was the
Tomorrowland Terrace Fireworks Dessert Party. Nothing could be "sweeter" than watching fireworks and enjoying some yummy treats with your daughter and her bff's. The nightly celebration starts an hour before Wishes Nighttime Spectacular begins at Magic Kingdom. You will need to purchase a separate ticket besides your park ticket to be admitted into the party. The party is very popular and only a limited number of people can attend so it is recommended to make reservations ahead of time. At the party you can enjoy an all-you-care-to-eat buffet of sweets and a spectacular view of the fireworks. The Tomorrowland Terrace Fireworks Dessert Party is a lot of fun and a great way to celebrate!
Something else that might be really fun to do is head to Downtown Disney for some bowling and sushi at Splitsville Luxury Lanes. Loud music, great food and bowling make this place awesome for teenage girls! After bowling you all could walk down to Basin, a luxury bath and body care store, and stock up on some fun spa products for your daughter. This store is a lot of fun, my 16 year old self would have loved to spend some time in Basin getting some products to take back to the resort room. Nothing is better after a long day at one of the Walt Disney World Parks than popping a bath bomb in the tub and soaking the day away! After Basin, hit up Goofy's Candy Co. for a beautiful made to order caramel apple or some chocolate covered pretzels and head back to the room for a full on spa evening! So fun!
Happy planning and happy birthday to your girl!