The unofficial "World's Smallest Park" is a square foot patch of grass that lies in a gap in the concrete outside of a storage facility in Oakland, California. This mini-park was built with a metal fence that can probably keep out 1-inch tall trespassers. It was also decorated with play equipment that looks like the perfect size for small insects. I wish every gap in the sidewalk had a tiny park.
According to the Guinness Book of World Records, Mills End Park in Portland, Oregon (above) is the world's smallest park. It measures 452 sq in.