Be sure the restaurant knows it is her birthday. They usually do something special like a dessert or flower, etc.
I am not sure your price ranges, but here are some ideas. . .
Romantic Carriage Rides at Riverside
Fireworks Cruises - IllumiNations or Wishes
Couples or Single Massage at GF or SSR Spa
Gift Baskets sent to your room or at check in through Disney Floral
Private Dining for two at GF
Room service private dinner at your resort (not sure about all
DVC resorts)
Parasailing at the Contemporary
Behind the Scene tours at MK, AK, or Epcot
Birthday Cakes (not sure of all the places with a bakery)
Cirque du Soliel tickets
I think it is possible to have a character sign a card or call the person too, but I am not sure how to do this.
Have fun and remember that anything special you do will be thoughtful!!
HTH
Ellen