I’m astonished that nobody has mentioned as easy as pie*.
It was one of the first things I thought of when I saw the question.
The Phrase Finder (www.phrases.org.uk):
"As easy as pie" is a popular colloquial idiom
which is used to describe a task or experience as pleasurable and simple.
The idiom does not refer to the making of a pie,
but rather to the act of consuming a pie ("as easy as eating a pie")
which is usually a simple and pleasurable experience.
The phrase is often interchanged with piece of cake,
which shares the same connotation.
Cambridge English Dictionary:
For Judy, getting a pilot’s license was easy as pie –
she seemed to have a natural talent for it.
Idioms by The Free Dictionary:
Extremely easy, simple, or intuitive;
requiring very little skill or effort.
After so many years as an accountant,
doing taxes is as easy as pie for me.
Boy, that test was easy as pie!
The Phrase Finder goes on to say:
What's the origin of the phrase 'As easy as pie'?
There are many similes in English that have the form 'as X as Y'.
These almost always highlight some property - X,
and give an example of something that is well known to display that property
- Y; for example, 'as white as snow', 'as dead as a dodo'
and, risking a group slander action from our noble friends,
'as drunk as a lord'.
How though are pies thought to be easy?
They aren't especially easy to make; I know, I've tried it.
The easiness comes with the eating - at least,
that was the view in 19th century America, where this phrase was coined.
There are various mid 19th century US citations that,
whilst not using 'as easy as pie' verbatim,
do point to 'pie' being used to denote pleasantry and ease.
'Pie' in this sense is archetypically American,
as American as apple pie in fact.
The usage first comes in the phrase 'as nice as pie',
as found here in Which: Right or Left? in 1855:
"For nearly a week afterwards,
the domestics observed significantly to each other,
that Miss Isabella was as 'nice as pie!'"
Mark Twain frequently used just 'pie' to mean pleasant or accommodating:
In The Adventures of Huckleberry Finn, 1884,
"You're always as polite as pie to them."
"So he took him to his own house, and dressed him up clean and nice,...
and was just old pie to him, so to speak."
Pie was also used at that time for something that was easy to accomplish;
for example, in The US magazine Sporting Life, May 1886:
"As for stealing second and third, it's like eating pie."
The Free Dictionary offers these alternatives:
- easy as falling or rolling off a log
- easy as ABC
P.S. Not One-Off Britishisms suggests that “easy as pie”
is the American equivalent of “easy peasy”.
I cannot find any indication that they are etymologically related.
Not One-Off Britishisms goes on to say
“easy peasy is now officially all over the U.S.”
I dispute this.
As I mentioned, “easy as pie” popped into my (American) mind immediately,
whereas “easy peasy” makes me queasy (and my spell-checker rejects it).
According to Google Ngrams, “easy peasy” is struggling
to become half as popular as “easy as pie” in British English,
and is barely even registering in American English:
* OK, somebody posted “easy as pie” in a comment while I was writing this.