He really liked history, particularly European history. There's some examples of this in his letters[0], but my favorite von Neumann story on this topic is how he managed to deduce the answer to a literary magazine's poetry trivia question when queried by his brother. They would both be teenagers at the time, and the magazine was in English, as opposed to their native martian, yet John got the solution at once.
Paraphrasing from here[1], although his brother's limited run biography/reminiscences has the story too.
The prize contest had the lines
They know this well my baron and my men
Gascony, England, Normandy, Poitou
That I had never follower so low
Whom I would leave in prison to my gain
I say this not as a reproach to them
But prisoner I am
"Did you know the poem?"
"No."
"Then how did you identify the poet?"
"Very simple" he said. "Gascony, England, Normandy, and Poitou were in one feudal hand only once during the early Plantagenets, and from there it was quite easy to associate with Richard's crusades and European captivity. But of course this is a translation, since quite obviously the early Plantagenets spoke Norman-Midieval French."
His brother then goes on to state: I found out much later that the translation was that of Henry Adams, and the Prison Song is only one of Richard's most perfect poems, usually referred to as gems of English literature!
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0. One to his daughter springs to mind, where he expresses fatherly concern about her getting married during her undergrad-- he's worried that she's too young and this will derail her career. After that's addressed, he then moves on to talking about one of her term papers, and proceeds to suggest an insightful take on-- I believe, I haven't read the letters in a decade-- some medieval French bishop. The juxtaposition of concerned father plus European historian in a man more known for axioms and automata was jarring.
1. "The Legacy of John Von Neumann" from the American Mathematical Society, https://books.google.ca/books?id=XBK-r0gS0YMC&dq (see pg. 22-23)