Oh there is LOTS to do after the parks close!!!
You can swim in the pools, you can go to DTD, Disney Quest, play Miniature Golf at one of 2 courses, go to Boardwalk, resort hop, have a late dinner, if it is the right time of the year, they show movies at Fort Wilderness, to Hoop de Doo, the Luau, a character meal, hop to Epcot and watch Illuminations.
Just because the 3 of the parks close, the magic doesn't end. That is the beauty of WDW!
Have a great time!