Other than all the theme and animal parks in Orlando, if you're gonna be near Kissimmee, there's lot of parks and fairs to hit at night. I liked Skull Kindgom...cool haunted house. There's also a Slingshot and Skycoaster in Old Town Kissimmee. Actually, all of Old Town is pretty cool.
There's this thing called Wonderworks in Orlando which is a big upside down house filled with all sorts of interesting stuff.
Those are the first things that come to mind, I'll put more if anything else popps in...
http://www.wonderworksonline.com/
Now, as far as night goes, I recommend the Universal City Walk and Downtown Disney Pleasure Island and The Disney Boardwalk as well.