I highly recommend San Antonio de Areco. I don't recommend it as a day trip, though, it's a full 2 hour ride (at least) from the centre of Buenos Aires and too nice and relaxing to just turn around and leave at the end of the day.

We stayed at an estancia which was pricey compared to what you'll spend on most of your Argentina trip, but worth it as a special getaway. When we were there in October it didn't feel too touristy - the area was very homey, cultured, and had this sort of old-world luxury and charm to it. But that was just before the tourist season began so I don't know how it is in summer (November-March).

San Antonio de Areco is an idyllic little town. Very well maintained with pretty historic architecture. The people there were very warm and a good number of them spoke English. I would have saved a bit more time for shopping, had I known there were some wonderful places to buy fashionable handmade clothes, silver jewellery and trinkets for the home. There are a couple of great cafes in the center of town and one near the church with amazing gelato. All in all, a lovely place to visit. I'd even recommend 2-3 nights there.

It would be handy to have a car in the area, but we found that the estancias will arrange for rides to/from Buenos Aires and into and around San Antonio de Areco, all for a flat rate that is probably worth it vs. the hassle and responsibility of renting a car.

One word of caution, though, ATMs seemed a bit unreliable in San Antonio de Areco so I recommend getting all of the cash you'll need before you leave Buenos Aires, and check how estancias accept payment at the time you reserve.