Celebrating a birthday at Disney is the best! Some activities will depend on how much time you have to plan and your budget, of course, but there are many different options!
--pick up a celebration "Happy Birthday" button for her to wear (you can pick up the buttons from Guest Services in the parks or - if staying on site - from your resort hotel). They'll even write her name on the button which will prompt observant cast members to wish her "Happy Birthday" all throughout the parks and could mean a few freebies from kind cast members!
--if you're staying at a Disney resort, speak with someone at the front desk and they can arrange for a special in-room Happy Birthday phone call from a Disney character.
--purchase a Mickey "It's My Birthday" MagicBand (possible option online if you order your MagicBands before your trip. Also should be available at the parks.)
--if you have a Table Service restaurant reservation (most you can do online which is highly recommended) booked for her birthday dinner, let them know you're celebrating and she might get a special free dessert (like a cupcake) or even a card. Table service restaurants will also let you have a custom cake (that you purchase) delivered to your table which could be fun after dinner. More info can be found on the Disney World website.
--not sure what parks you're planning on visiting, but MK, Epcot, and HS all have dessert parties during the fireworks shows that could be another surprise after a full theme park day. The parties are an extra cost (in addition to your theme park ticket) but they're fun - especially if your family has a sweet tooth.
--if you're hoping for a scenic view, a horse drawn carriage ride around select resorts is an option. More info can be found on the Disney World website.
--there are also two spas on Disney property that could be fun AND relaxing. More info can be found on the Disney World website.
Hopefully that helps! I hope your daughter's birthday is extra magical!
I'm all about a special meal for a special birthday. Something you wouldn't do normally. There are tons of good options at Disney. I'd recommend looking at some of the menus and see if there is a 'favorite' dish that one of the restaurants offers ( like my DH's is steak and lobster ). Custom birthday cakes can also be arranged for several dining locations.