It's a piece of cake is all about metaphors and similes.
Your own first language probably has many colourful expressions for everyday, and less usual, situations. Can you identify which ones we use in such circumstances, in English? 'It's a piece of cake' ~ i.e. easy, pleasant, even energy- and confidence-boosting!
The other two, upper Answers are good for expressing the idea of 'once in a very rare while' rather than 'never at all'.