We spent 3 days in Colonia - a one-hour ferry ride away from Buenos Aires- and loved it. It was such a sweet little town. We went on long walks, played on playgrounds and enjoyed some long lunches. It was a very relaxing part of the trip.
One of the craziest things about Colonia - a Uruguay delicacy called Chivita. It is a layer of French fries, a steak, ham, bacon, salads and cheese melted over the top. It was interesting.