It is a large brown bag, filled out with junk food.
Scooby-Doo! Mystery Incorporated
When the gang spied on Chen's coffee shop, Shaggy revealed to Mai Le (who was tagging along), Scoob and his snack bag, and described all it's unhealthy contents, suggesting they should eat the ice cream first. Moments later, the White Wizard attacked and took Mai Le, but she was saved when the gang fired the heavy snack sack from the Mystery Machine ejector seat and into the White Wizard.