Oh, Kelli, you are going to have the time of your lives at Walt Disney World with just you and your sister! It's really a whole lot of fun doing "adults only"! I went once with my mom and my sister-in-law and we acted like three kids the entire time! Well, except when we went to Jellyrolls!
Since you mentioned that you're staying at the Boardwalk Inn, you are going to be within walking distance of
Jellyrolls, a dueling piano bar right on the Boardwalk. It's a really fun place to go spend a few hours where you can put in song requests and sing along. I've even seen the piano players play games with people and it's a hoot!
My next suggestion is that you indulge yourselves with a spa treatment at the newly re-imagined
Senses - A Disney Spa at Disney's Grand Floridian Resort. They've got a fantastic array of different treatments in all different price ranges that can make for a very memorable and relaxing time to rejuvinate with your sister. I recently had a chance to tour the new spa and it is absolutely gorgeous! I'm confident that you will just love it! To schedule treatments, please call 407.WDW.SPAS or 407.939.7727 as far in advance as possible. Reservations are highly recommended.
I hope this helps and encourage you to come back and see me if I can help with any other suggestions!
Have a Pixie Dusted vacation!
~Jackie