Well it depends on what you have in your wardrobe! For example if you have some shorts and a hawaiian shirt you could nip down to the shops, buy some ice cream cones and some of that expanding filler and make a load of ice cream cones, then make yourself an ice cream sellers box that goes round that back of your neck.
Or buy yourself a toy gun and go as James Bond (if you have suitable clothes)
Or jeans and checked shirt means you could buy a cowboy hat and gun and holster.
Or do you have a suit that's slightly too small? A thin black tie, white socks and go down the market (most markets have a fancy dress stall) and buy a Blues Brothers hat and sunglasses.