Like the previous poster, I suggest spending the evening away from a park. There is SO MUCH to do without investing in another park ticket or adding park hopper (which will never be worth it for just one day of hopping!) Head to a special restaurant or dinner show (e.g., Hoop-de-Doo Revue, 'Ohana, the best restaurant at your particular resort, etc.). Alternatively, swim at your resort, go min-golfing at Winter Summerland (at Blizzard Beach, but doesn't require a ticket for that park) or Fantasia Gardens (near the Swan & Dolphin resorts), go resort hopping along the monorail and size up possible accommodations for your next trip. Shop, dine, bowl or see a movie at Disney Springs, f/k/a Downtown Disney (Raglan Road is one of many terrific restaurants there).
In a given week at Disney World, we typically spend at least two evenings away from the parks. (Plus, after a day at the unbearably hot and exhausting Animal Kingdom, you may welcome the chance to relax and take it easy!)