I know you get to open the park on the opening train. Also, you get to throw pixie dust just before the park is open to guests. Not sure about what happens after. I want to say I saw the family traveling with the Main STreet performers on the street car. Not really sure, though.
When I was younger, me and my family did it and it was the best thing I have ever ever done/chosen to do at Disney World! You can choose doing it becaused your asked to do it and we weren't going to say No!
We:
1. Opened the Park (Magic Kingdom).
2. Got an official certificate in a frame.
3. Got to travel in the front of the Monorail from the Ticket Station to the front of the Magic Kingdom.
Was an awesome experience! But we got there really early.