On cement or asphalt surfaces you could use chalk -- just draw your track outline. Or, use masking tape (be sure to clean up afterwards). If you have the permission of the property owner, spraypaint the track boundaries right on the grass or dirt or pavement -- not as easy to clean up or change as it would be if you used hose. If you have access to it, use powdered athletic field marking lime on dirt or grass surfaces.
An alternative that provides for even more creativity would be to use a waterhose or other flexible pipe or conduit. It makes it a lot easier to change your track design. For 1:8 and 1:10 scale vehicles, the larger 3" or 4" corrugated plastic pipe (used at a lot of RC tracks) helps to keep vehicles from crossing over the track boundaries so easily.
