I need help with an idea for a Harry Potter cake. It's only going to be a 1/4 sheet with chocolate buttercream. I don't know much about Harry Potter and the lady that I am doing the cake for is a huge fan. The cake and party are a surprise. So please give me some pointers.

You could make it look like a quidditch field or the sorting hat. There are a ton of HP cakes in the galleries. Here are some good ones I found for ideas:

http://www.cakecentral.com/modules.php?name=gallery&file=displayimage&pid=44721

http://www.cakecentral.com/modules.php?name=gallery&file=displayimage&pid=1359866

Here's a book of monsters from the movie:

http://www.cakecentral.com/modules.php?name=gallery&file=displayimage&pid=1225431

Good luck!