Day 1: Get metal trash like can lids and paint them to look like a shield. Get fabric and just throw together a crazy, bland, spartan dress or skirt (for men), also get paint stirring sticks and paint them like swords.
Day 2: Put everything on, hide in a bush, when someone walks by run out screaming "this is spartaaaaa!!!" - extremely fun.
From the editor: Excellent idea for a Spartan party!
Try adding decorations and maybe a choreographed Spartan sword fight at the party to spruce up the Spartan atmosphere!