http://youtu.be/iwB7W6gadtM Sometimes it seems like the coolest bike stuff is made for kids. Take the Chalktrail from Uncommon Goods. It's a really simple device that attaches a giant piece of chalk to the back of your bike. As you pedal, you leave a line on the road. You can make art and ride a bike, at the same time.
For some reason, the Chalktrail is shown only attached to kid-sized bikes. Which made me incredibly jealous. But a rep from Uncommon Goods explained that it also works on adult bikes. And I was lucky enough to receive one to review.
It's really easy to assemble and attach to your bike, as long as you don't use a quick release on your rear wheel. I biked about half a mile, from Bike Pretty Headquarters to Bike Handsome's house, and it did indeed leave a nice line.
The faster you go, the fainter the line. Based on how much I wore down the first chalk piece, I think you could only get about two or three miles from each piece. And be prepared for a distinct scraping noise. It's unnerving at first, but it doesn't mean that anything is wrong. You should just turn up the music anyway.
The Chalktrail for Bikesmight not be the best solution for marking a serious bike route, but it would be a lot of fun to take to a blacktop and mess around. The smoother and darker the pavement, the better the line quality.