When your guests arrive at the Harry Potter themed party, invite them in through a doorway which has been decorated to look like the brick wall at King's Cross Station's Platform 9 3/4. A plain sheet which has been painted and decorated using a sponge, paint and a marker will soon end up looking like the infamous brick wall. The guests will love this idea, as once they run through the gap in the sheet, they'll be heading straight into the magical world of Hogwarts!