If doing something is described as a piece of cake, it means it is easy to do.
Changing the wheel on the car was a piece of cake – I was happy to help!
I don’t know what all the fuss was about – the exam was a piece of cake! I had half an hour to spare.
The icing on the cake describes something that makes a good situation even better.
It was great to win the match but the icing on the cake was getting promoted to the Premiership!