Essentially Tinkertoys on steroids, as dreamed up by designer David Rockwell, these foam building blocks are part of an 825-pound, 75-piece mobile fun zone that will be popping up everywhere this summer. On May 29, the Imagination Playground in a Box arrives in the garden of the Cooper-Hewitt (2 E. 91st St., nr. Fifth Ave.; 212-849-8400; free on Tuesdays and select Saturdays through September). On June 26, an identical set will make a one-day appearance at the South Street Seaport, and on July 17, another will be unloaded on the High Line. It’s all part of Rockwell’s quest to promote unstructured play, the kind where kids slide down, bound over, crawl through, and amble across slides, forts, and tunnels of their own design.