Magical Greetings, Angela!
Welcome to planDisney and thank you so much for stopping by with your question. How wonderful that you will be celebrating a 16th birthday at Walt Disney World.  What a sweet way to celebrate her! There are so many wonderful ways to make her feel special during your stay. Here are a few of my favorites. 
Angela, the first thing I recommend is letting the Front Desk staff know you are celebrating a birthday, and to pick up your "I'm Celebrating" pins for the Guest of honor to wear throughout your vacation.  We did this when we celebrated our twins' birthday last year. Not only did they feel special by all of the "Happy Birthday" greetings they received from Cast Members and fellow Guests.  But, a few restaurants even brought them a surprise complimentary birthday treat!  It's not a guarantee that all locations do this, but a few do and it sure adds a bit more pixie dust to your visit.
Second, you can order a Celebration Cake shaped like a Mickey Hat for one of your special dinners. This can be added to many Dining Reservations at participating table service restaurants through your My Disney Experience account when making the reservation, and it will be brought to your table at the end of your meal.  Each cake feeds about 4-6 people, and is a great addition to let your birthday girl feel special on her special day.
Now, one of my new favorite experiences is the 
Rose Gold Afternoon Tea at The Cake Bake Shop at Disney's BoardWalk. My daughter and I did this on our last visit, and it is quite a special way to make your sweet 16 feel extra special. You can make a lunch or dinner dining reservation here as well, and you can order whole cakes from the Cake Bake Shop to have in your room. However, if you would like to book the Rose Gold Afternoon Tea, there is a concierge email and phone number to call and they will make all of your special arrangements. From name tags to personalized menus, this gorgeous restaurant is fit for any prince or princess!
Speaking of treats, a memorable family dinner at a special restaurant like 'Ohana or Story Book Dining with Snow White at Artist's Point at Disney's Wilderness Lodge is a great way to celebrate your birthday princess.   Alternatively, a 
Character Breakfast at Topolino's Terrace or Chef Mickey's is a dining event fit for royalty. 
Finally, I would recommend a unique 
Enchanting Extra the whole family can enjoy.  One of my favorite experiences is the Fireworks Cruise to enjoy Happily Ever After Nighttime Spectacular on a chartered boat on the Seven Seas Lagoon. It's a wonderful way to spend family time together, with an incredible vantage point of the fireworks after a cruise around the lake, and the music from the show is piped into the boat so it's like your own private viewing. 
Other Enchanting Extras I love are horseback riding at Tri-Circle D Ranch or a Capture Your Moment photography session. You could even give her a Disney Gift Card as a gift for her to use at one of the shops at Disney Springs to purchase a special Disney-themed souvenir.
Of course, the ideas are endless, Angela. But these are a few ways I recommend celebrating your birthday guest of honor and making magical memories of your trip.
I hope this helps you, Angela!  If you have any further Disney Vacation planning questions, stop back here to see me, and I'll be so happy to help you.
Magically Yours,
Keri