I've been to a Harry Potter-themed party, where were all sorted into houses and played things like beer pong and other party games, and scored house points. The host even had a House Cup for the winning house.

As fun as that was, I've never experienced anything that comes close to the theme park. It just feels like you are IN the world. So many little details, so many events, it's just perfect. I even turned in all of my cash for Gringotts bank notes and refused to pay with anything else.