Using cash at a fast food restaurant

At a restaurant where it is not possible or is more convenient to not use a credit card, have them break into groups of 4 and give them a $20 (or appropriate amount for the meal and location) out of a cash envelope. They are to return change to the person who gave them the money. Add up the change and subtract from the amount given out and this is the amount to put on the outside of the envelope indicating what was spent. It is best to capture this information immediately rather than waiting to when it is convenient or relying on getting receipts.