Fireworks! Disney's fireworks are AMAZING. Wishes in the Magic Kingdom, IllumiNations in Epcot, and either Fantasmic or Star Wars fireworks (possibly both depending on times) in Hollywood Studios. Not sure when you're going to be there, so I didn't include Rivers of Light, which is supposed to open this year at Animal Kingdom (probably December 31st, LOL...just kidding...kind of).
If you're not into fireworks/nighttime shows, then I suggest going to the Magic Kingdom and riding rides while others are watching Wishes. Lines should be shorter, and some of the outside rides are more fun at night AND you can catch the fireworks while ON the ride! Pretty neat.
If Animal Kingdom is open late enough, that's another park that could be fun. They have a "tree awakening" projection show on the big tree, and an evening street festival, and I hear Expedition Everest is really, really fun at night.
But seriously. Fireworks.