Right across the river, an hour ferry trip from Buenos Aires takes you to Colonia del Sacramento, the little town with historical quarter listed in the Unesco world heritage site. And it is worth the trip, once you take the ferry at the Puerto Madero Port in Buenos Aires, you arrive in an hour and a half - check the Buquebus or Colonia Express lines to see the schedule for several ferries per day- at the port of Colonia del Sacramento.
The historical quarter is walking distance from the port. As you go out the terminal and start climbing the avenue and then turn left towards the river on the main avenue of G.rales Flores, three or four blocks away on the left hand side starts the old town. the first place as you take the little street to the left will be the Sacríssimo Sacramento Basilica dating from the 17th century. Colonia del Sacramento witnessed power struggles between the Portuguese and the Spanish during the colonial era due to its geographical strategic location by the river Rio de Plata and was finally owned by Spanish and most of the buildings show the 19th century Spanish architectural heritage. The square in front of the Basilica is the Plaza de Armas squares, further ahead there’s a bigger one namely 25th May Square or Plaza Mayor, the big Square. On one of the narrow side streets of Plaza Mayor is located the Lighthouse, you can climb on top of it to watch the river and the view of the town on one of the two terraces (entrance fee can be payed in Uruguay or Argentinian pesos- in Colonia del Sacramento every place accepts USD as well).
Down on the street you can walk to the river side and climb on top of the oldest walls, Portão das Armas, where the first city walls were built, then continue to the port of the yachts and then walk to the other side of the river and go inside the Cultural Centre of Bastion del Carmen (has a nice garden looking at the river).
The little town of Colonia del Sacramento may not be as grand as Ouro Preto of Brazil, Trinidad of Cuba, Cusco of Peru, but it has such an atmosphere that takes you into a peaceful and timeless place, walking the streets over and over again you come to realise all the details that make it so beautiful, from the silhouette of the trees to the long sunset over the river, the quietness of the well preserved houses, and it is definitely worth a visit, even for a day from Buenos Aires.