As I typically want to avoid the Planet of Hats trope, I tend to believe that most of what makes up the average person doesn’t change much between race; especially in Warcraft where most of the playable species are mostly very similar to Humans (irl, I mean) socially and behaviorally.
In the case of longer-lived races like Night Elves, if I may indulge in a somewhat relevant tangent, many tend to chafe at the thought of properly portraying people who naturally live significantly longer than humans typically do. I’ve long seen this as a somewhat overblown concern. It largely depends on the individual. Anyone living for a long time is bound to have some degree of wisdom, but how their time is actually spent is just as if not more important. A Night Elf who spends their years living like a moronic stoner is unlikely to be less of one if they lived for a millennium versus a decade. But I digress.
When it comes to various smaller matters, I imagine its best to simply rely on one’s own powers of speculation in most cases. To my knowledge there isn’t really a widely accepted headcanon for most if any of it, but I admit that, despite being a big fan of the race, I’m far from the most knowledgeable. I know enough to know that one can’t logically be centuries old and born on Teldrassil, at least by natural means, but I imagine there are others knowledgeable who might recall ancient factoid or some such that could help.
Speaking of factoids, I’d like to share a couple, including one or two things about my own characters, that I remember or otherwise thought relevant enough to mention:
The Sentinel Army originated from the Sisterhood of Elune, replacing the previous Imperial Army (as I personally call it), which was noted to be corrupted by nepotism (https://warcraft.wiki.gg/wiki/Sentinels#Early_history)
At least at one point Night Elves were characterized as being nocturnal. I highly doubt the devs still acknowledge this, but I personally do.
Many have also at one point or another characterized Night Elves as being xenophobic, though I presently can’t be bothered verify the veracity of that.
The Warcraft Wiki page on Night Elves also includes a bunch of stuff on their culture, if you haven’t looked at that yet; including a small section on cuisine (https://warcraft.wiki.gg/wiki/Night_elf#Cuisine)
One of my Night Elves, Phylande Lightriver, possesses a great interest in machinery. I tend to think of this as somewhat abnormal, but perhaps also understandable for one with limited (if any) access or exposure to such technology.
None of my Highborne characters have facial tattoos as I presumed that wasn’t a cultural trait practiced by the caste, due to no Highborne npcs (save Azshara) having any.
My Highborne characters are also generally more at home in urban areas as the race, or at least of that caste, seemed implicitly more urbanized before the War of the Ancients. Conversely one of my younger Kaldorei characters is significantly more comfortable in natural areas, having spent most of her life in such places.