It is very easy do go on a day-trip to Colonia del Sacramento from Buenos Aires: a one-hour ferry ride (around 25$ one-way) brings you to the other side of the River Plate. Colonia is a charming yet quiet old town in Uruguay founded by the Portuguese in the 1600s. It is extremely walkable, and all sights can be covered in an hour or two.
One thing we noticed is that almost everyone drinks Yerba mate (a tea-like drink) religiously, even carrying a separate matera bag for it. The area around the old city wall offered an artisian local ice cream: helado, which was delicious. Apart from the church built on remainders of the city wall, the many old-timer cars were another highlight of the town.
It is a popular weekend-destination for foreigners and Argentinians as well. Moreover, because the town is located in Uruguay, its ATMs are very popular as they dispurse U.S. Dollars. More about why this is great for foreigners in this post.
