First, many thanks to those on this forum who offered advice before and during our second trip to Buenos Aires, from which my wife, son and I have just returned. The reason this trip report is somewhat thin is because we didn't, by design, do much of anything on this trip that merits a report!
For the most part, we just watched the world go by, explored neighborhoods, sat in parks, poked around little shops and markets, and ate in restaurants chosen on on the spot... essentially soaking up the vibes of the city in a more relaxed mode that we did three years ago. For us, it was the perfect city holiday. (0K, it did get a bit warm there a couple of times!).
The only observations I can offer that conceivably could be of interest to anyone contemplating a visit are:
Arrival & departure: Used Taxi Ezeiza to and from airport. No advance reservation on arrival and booked online for return. Easy.
Apartment Rental: Used Reynolds Propiedades. Smooth and professional in every way. Would recommend them. Comprable with the positive experience we had last time with BA Habitat.
Neighborhood: As has been discussed on this forum, the Palermo Botanico neighborhood has a lot going for it. Not as posh as Recoleta, perhaps, nor as trendy as Palermo Soho or Hollywood, but a very pleasant mix of residential and services, and easy in which quickly to feel at "home." Thanks to Marnie and Ricardo for pointing us in that direction.
Prices: Noticeably higher on a broad range of stuff than I remember from late 2008. Still a refreshing value compared with, say, Europe, but clearly no longer the breathtaking bargain it was. Parenthetically, and maybe I missed it, but the black market dollar exchange option was not as evident as I had for whatever reason imagined.
Lujan: Took day-trip to Lujan, primarily so my rail enthusiast son could ride the local train (out of Once station). Let's just say we're not talking the Eurostar here. In Lujan, the Basilica is impressive enough, I suppose, but this outing probably needn't rise to the top of a visitor's must-do list. Incidentally, I had in Lujan not only my worst meal in Argentina, but maybe the worst meal in my life! We had lunch in what seemed to be the most popular place on the square, a bustling parrilla buffet libre. Truly inedible meat. At least I think it was meat. The locals were chowing down enthusiastically, though, so go figure.
Running: In addition to riding trains, my son is a runner. For him, the Palermo parks, as Scarlett suggested, were wonderful. Also, if any runners happen to be in BA over new years, the San Silvestre 8K race, for which they close Av 9 de Julio and some other big streets downtown, is something to consider. Easy to sign up online.
Montevideo: Side trip to Montevideo was a highlight for us. Very pleasant, almost laid back city (at least compared with BA, and at least in the Pocitos area). Stayed in Pocitos Plaza Hotel. Not the Sheraton, but perfectly comfortable, ideal location, helpful staff and, at USD $110, a pretty good deal. Dinner at excellent parrillada recommended by the hotel called La Otra. Lunch at Mercado del Puerto. Place was packed with cruise ship day-trippers but still good fun and wouldn't have wanted to miss it. Buquebus fast ferry is the way to go. We took the bus/boat combo back and wished we had done the fast boat both ways.
That's about it. All in all a great trip and we look forward to the next one. Thanks again to the DEs for your guidance, and especially to Verobaires for the helpful PMs on various topics.
PS - My son and I quite enjoyed meeting Marnie & DH and being the first to hear the story of their' shall we say, not-so-smooth landing.