Hey,
I've had many birthdays in Walt Disney World. The one tricky thing you mentioned is basically that you're ending up where I always start off.
There are deffinately some great things to do in the Magic Kingdom, lunch in Cinderella's Castle certainly comes to mind.
But my suggestion would actually be to celebrate your 24th birthday in EPCOT Center. Have your day culminate in Dinner at the Morocco Pavilion. You wander back through their Medina area... Have an exotic meal.... Be entertained by a stunning Belly Dancer... and top it off with the Illuminations Fireworks Display out at the World Showcase Lagoon.
Now that is if you want to stay in a park. Perhaps the best place on the property to have a personal celebration is The Adventurer's Club. I'm a charter member.... Congalush!! to my bretheren out there.
Located on Pleasure Island, The Adventurer's Club is a one of a kind experience. Here the goddess Babylonia can help you celebrate your Birthday in style, while you lift a Yakuz Mug to yourself in the company of Samantha Sterling, Hathaway Brown and many other brilliant impovisationalists who work not on stage but among the people in the bar.
Don't miss the shows in the Library, starting every half hour. Each one is different.
Stay long enough and they will let you go through the enduction ceremony!!
I hope there were some good ideas here.