We have the disposition for late-night fun, so we're pretty excited about that.
Then, La Recoleta is for you. It's an area near a well-known cemetery in BA (which is, of course, La Recoleta). Life starts at around 11 PM or later; lots of restaurants open and serving incredibly good fare... at incredibly high prices (not exorbitant, mind you), and a nice, festive environment.
La Recoleta is the upper-class cemetery, and lots of diplomats, writers and famous people are buried there. The other cemetery, representing the opposite side of the social spectrum, is La Chacarita, in which other writers, poets and actors are buried. From both, only La Recoleta has the glittery spirit of the place to which one goes "to be seen."
When we heard from the hotel owner about La Recoleta, we went just to check it out. We ended going there twice, because the shops, restaurants and bar are open and doing brisk business.
Hence, don't be apprehensive at all. BA, like Barcelona and many European cities, can be a dangerous place if you're unfortunate (wrong place, wrong time). If you just act as if you were in Boston, NYC or any other large city, you'll do fine.
BTW, La Recoleta is a great people-watching place at any time of the day. If you can do it, go to "La Biela" ("connecting rod") for an afternoon coffee: it's a cafe-restaurant with a lot of class and tradition. Apparently Juan Manuel Fangio and other drivers either used to meet there, or had something to do in its inception, hence the automotive-related name of the place.
Take care!
🙂