Thank you for sharing the article. I found that it reminded me of another resource about thinking, a video by RSA Animate which takes Sir Ken Robinson's talk on Changing Educational Paradigms and animates it. If you haven't seen the video it's quite interesting. I use this video to introduce creativity to admins and teachers. Animation can be a powerful exercise in representing ideas and information. If you have seen the video, start at 7:46 to review what divergent thinking is and how it relates to thinking and creativity.
Back to your questions about how it relates it to PBL and whether there is a place and time to give students a chance to "play" and explore in school. In my world, project and problem based learning has an element of discovery. In discovering answers children should have a chance to play. It's through imaginating answers in their heads that learning happens. The corresponding proving or disproving process is the process by which they learn. They evaluate, analyze and come up with a hypothesis. This is what makes the discussion about informal and formal learning so interesting as well.
In your work with PBL, is there an element of "play" and how it is typically executed?