Back in high school when I was campaigning for student body president I thought it was such a GENIUS idea to use sidewalk chalk advertising. It's cheap and people notice it. Some down sides are it's a little messy, and if the weather is bad- you're out of luck. But what if you lived in sunny california? The coastal boardwalks provide ample foot traffic where your ads would get noticed.
Water stenciling, also know as "reverse graffiti" or "green grafitti" is a fairly new form of advertising that allows brands to get their ad across by virtually power washing sidewalks. People use high-pressure water guns and stencils, which cleans the sidewalk and provides a new ad. Some of the first companies to use this form of advertising are GreenGraffiti (Amsterdam), CURB (London), & GoGorilla Media (New York).