Lisa can possess a sheet!

Christoph can literally be ANYTHING he wants. I'm thinking he'll go as a coffee table.

Lisa doesn't like parties, she'll probably only go if Ev goes, and I doubt that.

Grace will go in a giant mech. [s]What? It's a COSTUME! What ABOUT the laser turrets?[/s] Scratch that, Grace will be going as... any one of a million half robotic fictional characters.