To have the icing on the cake means to have something additionally good to what you already have. It refers to the fact that many people love to get a piece of cake with the delicious icing on top.
1. Rohit achieved his goal of becoming a Diamond level member at ISC and the Wizard of the Month award was the icing on the cake.
2. Meenakshi's application to Harvard University was accepted and it was an icing on the cake as she had already got her visa approved without the usual hassles.
3. After winning player of the tournament, getting the prestigious Rookie of the Year award was the icing on the cake for the young golfer.