Dig up a good pile of dirt and use the hose to make mud!
Transport the mud to the driveway in buckets or pails.
Use old paintbrushes to paint with the mud on the surface of the driveway.
It's a good idea to ask parents first, wear washable clothes AND clean up your mess when you are done.
To clean up - just hose down the driveway! This can be as much fun as making the mess!