I ate at a Thai restaurant last week and the credit card receipt was annotated with some hints for what to use for a tip. The numbers were automatically calculated, so that it was customized for the amount of the bill. Just below where the customer writes the tip, it had:
15%=$5.00 18%=$6.00 20%=$6.67
This is great because most people probably give between 15 and 20% but can't calculate it quickly in their head. So with this receipt, the customer saves time, effort, and is more likely to put down the correct amount for the tip. Great idea, yet so simple.