to get ideas for the costumes, go to historical or museum web sites
Campers and staffers dress up as Greeks (togas and beards), Romans (tunics, sceptors/swords and olive-branch crowns) or egyptians (anhk necklaces, cloth wraped around your waist like a skirt, posterboard crowns). Stage a three way game like ambush between the empires. Also, carry the camp director on a folding chair lashed to 2 long, thick dowels and declare him your king.
Go to historical or museum web sites to get ideas for the costumes.