Here's a way to show off your Halloween costume around town: by participating in this Sunday's Barbary Coast Trail Walk, where flamboyant Grand Marshal Mayor Willie Brown (who, along with other guides, will be in Barbary Coast costumes) will lead you through the city's most historically vibrant neighborhoods. Participants in this inaugural 3.5-mile walking event will meet at Mint Plaza at 9 am and walk through downtown, Chinatown, North Beach, the Embarcadero, then clear on down to the water.