In Berlin are a lot of empty construction areas ... plain , dirty, unused land that is staying like that for years. Why not to plant seasonal flowers, some vegetables or simply grass in that areas .... also make some simple cheap urban furniture ... or ... make some events there ... this idea is about giving this spaces back to the public as temporary urban interaction areas (as long as the owners are not decided to built something there soon).

Already happening in Berlin: http://prinzessinnengarten.net/about/

Other approach: http://www.guerrillagardening.org/