One of the joys of english in particular is how words and phrases change meaning over time. Look at the word 'gay' for example - 50 yrs ago (or less) it only meant 'happy' and now it has a totally different meaning.
The language you use also reveals a great deal about your age, nationality & social status.
How about the following:
As easy as pie
It's a walk in the park
It's a breeze
As easy as taking candy from a baby