My daughter and I just did a girls-only Sweet 16 trip this summer. You don't mention when this trip is happening, but ours was in July. I agree with previous posters, the best things to do on girls-only trips are special meals and spa treatments.
We had sit-down lunches every day (mostly to get out of the heat in July), but it was so nice to sit in a (sort-of) quiet, air-conditioned, usually dim-lighted restaurant for about 60-80 minutes everyday. I really recommend Le Cellier at the Canada Pavilion in Epcot, Be Our Guest in Magic Kingdom, the California Grill in the Contemporary Resort, the Sci-Fi Dine In at DHS, the Yak & Yeti in Animal Kingdom, and Cinderella's Royal Table in Magic Kingdom. I also booked 2 character meals. I wasn't sure that my 16-year-old "adult

" daughter would care about characters and pictures, but she jumped up from the table at every opportunity and took pictures and selfies.
As a note, many of these restaurants I booked at exactly 180-days out, and scheduled my park days around meals. This might not be everyone's preferred method of, and level of, planning. I had also purchased the Table Service Dining Plan, so many of these meals fell under that plan, and I paid out of pocket for a couple of them. This was a special, never-to-be-repeated trip, and I admit I really splashed out on the meals.
The other thing we did which I thought was really fun and special was a park-free day at the Grand Floridian, including spa treatments and lunch at the Gasparilla Grill. Neither of us had ever been to the GF, and we did this on the 6th day of our trip as a refresher day. I had a massage appointment in the morning, and my daughter got to sleep in as late as she wanted. I had her ride the monorail over to the GF from the Contemporary to meet me at the monorail station on the top floor of the GF. This was conveniently located next to the shopping, and we poked around a little, and then went down to the Grill. It was fun to try a really casual lunch at another resort. She had her spa appointment after lunch, a Signature Pedicure. There are a few requirements for under-18s doing treatments, and one of those was that I had to sit in the salon with her as she had her treatment. No big deal, they let me sit in the pedicure chair next to her and I read my Kindle.
My biggest recommendation for going with teens, especially girls, is to try to work off their schedule. I learned really quickly to give my daughter down-time, and a few sleep-in days, and extra time to get ready in the morning. Because we were not traveling with little kids or her brothers this time, we started our days early, used FP+ to our advantage, and had done many of our big items by lunch. We had those nice sit-down lunches, usually did one more attraction, and then went back to the resort for down-time and rest. Big kids like to be out late at night, so we went back to a park for dinner, and closed the parks most nights, including a 3:00 A.M. night in the Magic Kingdom!!! That was fun!
You will have an amazing trip, and it's so fun to travel with grown-up kid!