Hi Madeline,
Happy early birthday to you! What a special milestone to celebrate in such a magical place. I, too, celebrated my 21st birthday many years ago at Walt Disney World and will never forget it.
One of the things I did that was so much fun was go to
Jellyrolls, a dueling piano bar at Disney's Boardwalk. This lively joint is full of great tunes and is sure to deliver on a fun evening out. The piano players take requests and have been known to pull up those celebrating birthdays for a special serenade! Keep in mind that this place does charge a cover and is quite popular, so if you don't get there early, you might be standing until a table or barstools become available. But, I can say from my many experiences there, it is well worth it! Also, it is very close to the Yacht Club...just a short 5-10 minute walk across to the Boardwalk, no transportation needed (unless you want to take the boat there or back).
Some other unique and memorable experiences you might consider are
La Nouba, a Cirque du Soleil show or
Characters in Flight, a tethered balloon that soars 400 feet in the air. Both of these extraordinary activities are in Downtown Disney and do require separate ticket purchases from park admission.
May your birthday be full of wonderful memories to last a lifetime and don't forget to ask for a birthday button when you check in. Be sure to wear it for the duration of your trip. You never know the kind of special attention you'll get from it!
~Allison L