Buy the Cinderella Slipper cake by calling WDW Bakery and take her to the California Grill for dinner. Make AR during the time you would be viewing WISHES over the Magic Kingdom with dessert.
For my DW's 40th, we took a birthday fireworks cruise. They supply snacks, sodas and a cake on a pontoon boat decorated with balloons. the boat tours in front of the MK with a great view of the fireworks. As much as she hated turning forty, it took some of the sting out of the day.