I'm generally not too fond of visiting cities and spending a lot of time in the city, although there are some exceptions occasionally. To me, Buenos Aires was not that appealing, but what is nice about it is that you can relatively easily take a day trip to Colonia del Sacramento, in Uruguay. A quick 1hr ferry ride across the Río de la Plata is all that it takes.
Depending on your belief, the Rio de la Plata is either the widest river in the world or a small gulf. Either way, at up to 140mi across, you won't see the other side. This beach seemed pretty nice and inviting - hardly anyone around and it was totally beach weather!
Travel Tip: When it comes to booking a ferry to Colonia del Sacramento, you have a few choices. There's Buque Bus, Sea Cat Colonia and Colonia Express. Generally Buque Bus is the most expensive, so try the other two. There are both slow (3hrs) and fast (1hr) ferry rides. Often times from what I heard, the Sea Cat ferry is exactly the same as the Buque Bus one, but for a lower price. The rate is also cheaper if book further in advance. I ended up using Sea Cat which was actually a Buque Bus ferry in the end.