There are three suggestion boxes in New York located in Park Slope, on the Manhattan Bridge, and at Stanton and Allen. You can see the suggestions collected so far on the site here.
Pretty cool idea. I think people could use a bit more guidance though–asking for suggestions on a particular thing, theme, or aspect of the city.
My friend Abe did something similar called I Am Awesome Because. He posted paper and pen with the prompt “I am awesome because…” and people filled in the blank lines with everything from props to a wife to sobriety.
Just this morning I got an email from friends about all the summer activities we can engage in including lemonade stands, baseball, picnics, and water balloon fights. Maybe we will run a summer fun suggestion box, too.