Is it your birthday, or the birthday of a child? If it's a birthday for a grown-up, you probably can't go wrong with a few adult beverages and a nice dinner in World Showcase.
I love fireworks shows, so any birthday that ends with a nighttime WDW show is good in my book. With that spirit, dinner at California Grill with its second-to-none view of Wishes may be nice. DH really liked that a few years ago.
Are you a kid at heart? Maybe CRT for some girly princessy time?
On one of my birthdays, I took the KTTK tour, which was pretty fun, but if I did a tour again on my birthday, I'd do something less informational and more plain fun, like a Segway tour, or snorkeling in The Seas.
Another thought is some spa time. Long story short, you'll have to pay for whatever experience you choose, but there are lots of ways to make your birthday even more memorable.