Hi, Kaila! My daughter, who is 21, spends a ton of time at Walt Disney World, and often goes with friends who are your sister's age. They love to hit the attractions that her younger sisters aren't as excited to do, such as Space Mountain or Expedition Everest. They also love going to the characters to get photos taken. You would think this was for younger kids, but they really love having these pictures to commemorate their trip.
You may want to look into some of the specialty tours. There are Segway tours that are very popular, or you might want to check out the Wild Africa Trek that is really a great behind the scenes tour. You can find information on some of these experiences by check out
Disney World Special Tours and Experiences.
Dining is another thing you can do that steps up a trip to a more "grown up" experience. Book yourselves a nice dinner at the California Grill or Jiko. The meals are at a leisure pace, so you won't find as many children there as you might some other restaurants. Or check out Raglan Road Irish Pub, where you'll find good food and live entertainment each evening.
Have an absolutely amazing time!