Under the sea comes in a close second. I treat the kids like they are older than what they are. For example, the curlers and nail polish at the slumber birthday party were a BIG hit. None of the girls had actually worn curlers before, and they were all involved for at least two hours wetting hair and wrapping it, then letting their partner do their hair. Next came the actual styling.
Kids love to feel like their special and grown-up.
I never plan anything that is physically challenging one child against another, such as a relay race, that would make someone feel inferior. Buy a year in advance. Stay away from themes that follow movies when you buy in advance, because what is popular this year will be totally out for next year, by the time the next Disney cartoon comes along. Fun doesn't necessarily have to be expensive. Household items can be turned into crafts.