When our kid was in preschool, we made this easy Pilgrim costume the night before his Thanksgiving party with things we already had on hand.

We painted this hat black and attached a piece from a yellow foam sheet to make the buckle.  We made a similar buckle for the belt, which we made from a piece of black scrap fabric.

Add long white socks (or cut off a shirt to make some faux long socks) and you're done!