This TED Talk from designer Tim Brown is awesome -- it's about how playfulness is crucial to creative thinking. And it really is fun: at one point in the talk, he asks audience members to bombard him with foam missiles, leaving the stage littered with drifts of toys. At various points he asks the audience to draw pictures and other playful things. He also goes through various examples of how play has led to important designs in the real world.
Discussed: how adults fear the judgment of our peers, how kids don't (at least when they're young), forming a company consisting of your best friends, how Pixar and Google employees work in playful environments, the Finger-Blaster (!), tin foil and what you can do with it, how we self-edit as we have ideas, mescaline, the Purdue creativity test, designing a solar space probe while high, Eames experiments with plywood, how roll-on deodorant led to the first commercial computer mouse, and much more.
Here's the video. The quality is poor, so you may want to watch this high-resolution version instead.