Coroplast signs are those seen on the metal post that stick in the ground.  Great for identifying areas such as parking, concessions, etc.  Also many of these signs are seen during elections.  These signs can be printed on one side or both.  Popular sizes on these signs are 12"x18" ($19/$24); 24"x24" ($39/$49).  Signs can be made as large as 48"x96". 

One tip for this type of sign is to make them as generic as possible.  These signs are very durable and can be used over and over is taken care of when storing.