My friend said to me: " So, there are two things I hate in the world. One is liars, two is giraffes." I say the proper subject verb agreement is "a liar" but he wanted the listed items to be plural. What is the grammatically correct way to say this or was it acceptable to begin with?
I interpret things, as used in your quotation, to mean "categories or kinds of things"—and certainly liars can be a category label, as can giraffes. Based on that interpretation, here is how I understand the sense of the full quotation (with the implicit words made explicit):
So, there are two [categories of] things I hate in the world. [Category number o]ne is "liars," [and category number] two is "giraffes."
This is not to say that the speaker couldn't have used your preferred wording:
So, there are two things I hate in the world. One is a liar, two is a giraffe.
But in that case, unless the speaker has a particular liar and a particular giraffe in mind as the objects of his ire, the sense of the expression is very similar to the one I gave for the plural forms:
So, there are two [categories of] things I hate in the world. [Category number o]ne is "a liar," [and category number] two is "a giraffe."
The phrasing is grammatical whether you use liar and giraffe or liars and giraffes.
This is an interesting question, because there is a good movie that uses a similar line. If you ever saw The Good, the Bad and the Ugly, there was a few times in the movie where characters say, "There are two types of people in this world: those who _____ and those who _____". They always use plural to mention the two options.
If I was writing your sentence I would say, "So, there are two things that I hate in the world: liars and giraffes." I would use plural to avoid the reader believing that you hate some specific liar and some specific giraffe.
P.S. don't look up the quotes from The Good, the Bad and the Ugly unless you want spoilers that you shouldn't want - it is one of the best movies of all time, which is one reason I trust the grammar of it. Sergio Leone, that movie's director, has never steered me wrong before.