Two dumpsters full of greenery, with four more to come, add a bit of nature to the streets of a paved-over downtown neighborhood. Some scoff, but others are willing to give the 'parkmobiles' a go.Reporting from San Francisco -- The greatest park in San Francisco arguably is Golden Gate — 1,017 sweeping acres studded with playgrounds and windmills, lakes and museums, a Shakespeare garden, a brew pub and its very own herd of bison.