Check out this cool concept for bikes: Contrail.  Basically you attach their device and ride around, leaving a colorful trail of (non-toxic) chalk.  It makes biking more interesting and fun while also allowing the trail to act as a guide so people don’t get lost if you’re in a large group of different skill levels.  Pretty cool!

A demo of the trial version of the contrail device was shown at the Opportunity Green Conference in 2010.

As you bike, you leave an imprint on the road. It's non-toxic and washes away, but doesn't wipe away so easily.