Here's a couple things I've done with standard kids' sidewalk chalk. No plans or reason; mostly just for fun. It's also been nice to create something without any agenda. Crummy commercial chalk on a gritty city sidewalk is such a low-pressure situation that I can more easily be in the creative moment - no worrying about quality or focus or client bullet points. Chalking these designs helps to remind me, on a very basic level, to listen to a different voice.
It's tough on the back, however.