Colonia del Sacramento
As its name would indicate, Colonia is a colonial city. Not only are there beautiful beaches in this area, but there is also a lot of history to explore. This location is in Uruguay. However, it is fairly quick trip to these beaches from Buenos Aires, as you merely have to cross the Rio de la Plata by boat. Some of the popular beaches in Colonia del Sacramento include Atlantida, Piriapolis, and Punta del Este.