Help needed for HP role-playing game

[Anne]

My daughter and I will be attending a "family weekend" sponsored by 
our church September 12-14. In an effort to provide a somewhat 
unusual activity, I (perhaps foolishly) offered to lead/coordinate 
a "Harry Poter role-playing game". I told the program chairperson 
that I would need only 1 hour for this game. My idea was that anyone 
of any age who had read or listened to the books could participate. I 
would write the names of characters on pieces of paper and put them 
in a hat (perhaps the Sorting Hat isn't busy that weekend??) and 
people would portray whoever they pulled out of the hat. I am 
guessing that we could have anywhere from 3 to 20 people involved in 
this.

Now, my big question is this. Has anyone done something like this 
before? Should I attempt to write out a skeletal plot for people to 
try to follow? Or perhaps a starting point and an end-point that we 
could try to reach in one hour, and let the participants figure out 
how to get there *in character*? HELP! HELP!!!

Muchas gracias,
Anne U
(who might need gillyweed if I get in over my head on this)