Chalktrail is easily one of the neatest things I've seen for summertime fun activities so far this spring. It's really simple: just clamp on the device (it comes in two models for either bikes or scooters), insert some sidewalk chalk, and begin creating art while you exercise. It's even supposed to be so easy kids can do it without their parents help, since it requires no tools! The device is a flexible wishbone, so it'll keep the chalk pressed to the ground on either even or uneven surfaces. It will only hold one piece of chalk at a time, but the possibilities are limited only by your own creativity.
And yes, it it adjustable for adult-size bikes, too. Which means the whole family can get out and spend some of that fabled quality time together while getting some exercise and creating art. There really isn't any losing here that I can tell. The project was recently funded by Kickstarter, and there are ongoing talks with "a major toy company", so this should be hitting the shelves pretty soon. It does have its own special chalk (each piece should last about 1.5 miles), but I bet you could probably make it work with the stuff in the huge bucket from the dollar store. Either way, it's going to be pretty darn cheap. Proposed pricing is only $14.99 US for the scooter model and $19.99 US for the bike model.