My three things...
1. definitly the fireworks, and night time spectaculars at Disney
2. just the whole essance of the characters, go to one character dining, maybe Chef Mickey's and just pretend you are a kid.
3. take the time to visit all the countries in World Showcase, lots of neat stuff there to do and check out, and stuff to buy.
There are so many, i too agree three is hard to come up with