Hello Dawna,
Nothing is more magical than spending your birthday at Disney. Your cousin is one lucky gal!
My first recommendation is to make sure you pick up a complimentary celebration button at Guest Services when you arrive. Tell your cousin to wear the pin proudly throughout your visit. It lets everyone know about her big day - and first visit. You never know what Pixie Dust they may be able to share.
Have you considered hanging out on the
BoardWalk during your vacation? The BoardWalk comes to life at night with street performers and carnival games. And it is lined with food vendors with cheap offerings. And if you want to splurge a bit, you two can rent a surrey bike and ride along the pier.
Another thing you might want to check out is the Electrical Water Pageant. I love grabbing a snack then watching the "parade" float by from the beach at the Polynesian Resort. There's just something about the tiki torches along the paths at the Polynesian that I just love! All of this for the cost of your snack.
I hope this helps. Enjoy your trip!