The Khlong Toei district in the heart of Bangkok is packed tight with buildings and ­people—which doesn’t leave much room to build new parks, let alone giant rectangular fields on which kids can play soccer. So real estate firm AP Thailand took a different approach. As part of a recent project, the company used aerial photography to find what developer Pattaraphurit Rungjaturapat calls “untended areas,” or unusually shaped patches of land that weren’t being used. Then it covered them with concrete, paint and anti­slip ­materials—all the trimmings of a proper sports venue, without the typical boundaries. Not that locals seem to mind: Rungjaturapat says the first two fields, which opened earlier this year, are packed with kids as soon as school lets out. This December, AP Thailand plans to open a third.