To have nobody to answer to in Disney World but yourself! Sounds like fun!
Treaet yourself to the SPA!. I don't know where you are staying, but check out this
PAGE on the Disney website that will help you find a convenient location.
Head to
Downtown Disney for the day. Pick a restaurant, grab lunch, and eat outside while enjoying the Florida Sunshine! One of my favorites is Wolfgang Puck Express pizzas and salads. While in Downtown Disney, you can shop at all the fun stores or even catch a movie.
Since you are engaged, you might enjoy stopping by "Francks Wedding Studio" by the Wedding Pavilion. (Located next to the Grand Floridian.) Here, Brides-to-be can plan their Fairy Tale Wedding. I always like to peek at the lovely cakes and gowns they have on display!
You can go to the parks! Pick attractions YOU want to go on, enjoy the beautiful gardens, or take a
tour at the Magic Kingdom or a
tour at Epcot. We did the Keys to the Kingdom tour a few years ago and enjoyed it.
I am hoping your fiance has a few hours in the evening to spend with you. You can go to the Polynesian Beach to watch the Magic Kingdom fireworks. (Check ahead for schedules.) Or, go to Beaches and Cream for ice cream and stroll the Boardwalk. If you time it right, you'll be able to see fireworks from Illuminations at Epcot.
If you think you'll be able to have dinner together, make dining reservations. Some suggestions: California Grill (Contemporary); Jiko (Animal Kingdom Lodge); or Yachtsman Steakhouse (Yacht Club).
Have FUN!