Our daughter is turning 16 in WDW this trip, too. We are having her celebration at California Grill and will be there for fireworks! I have gathered on these boards that custom cakes can be made at some restaurants, and they seem best from CR, so that's one of the reasons we chose this restaurant on this night.
Some ideas we had for the day were tea at GF, a character breakfast, lunch in the castle, a spa treatment...she picked Typhoon Lagoon for the day, so now we are leaning toward a "down day" plus a nice dinner. Another great venue for a party is HDDR. If you've never done that, it is wonderful for the whole family, and very festive. A fireworks cruise might be nice too, we've never done that.
Good luck planning! I'd love to hear what you decide.