Gilbert isn't a stranger to pranks. Of course the pranks are generally harmless, but they're gonna have to be elaborate. Gilbert is generally good with casting charms and jinx on objects so maybe I suggest a "Sorcerer's Apprentice" type prank? With enough preparation before hand, the prankee's won't know what hit them. Will definitely freak them out if all the stuff starts coming to life and maybe even starts talking to them?

He's already dragging Ludwig and possibly Romano into the plan, but any extra help would be great as well. Its gonna be a long night for preparations.