Colonia del Sacramento is a charming small town in Uruguay and the perfect day trip from Buenos Aires. With its cobblestone streets, historic buildings, and mix of Portuguese and Spanish influences, it is the perfect place to relax after a busy few days in Buenos Aires.
There are a few museums, a lighthouse, and excellent restaurants on the water. For something local, eat a chivito sandwich with all the fixings for lunch while you're here.
There isn't much to do in Colonia and that is the charm. It's peaceful and a nice change of pace from the busy city.
You'll take a ferry from Buenos Aires, passing through immigration for both countries before departing. When you return to Buenos Aires, you'll once again pass through both country's immigrations on Uruguay's side before leaving. So, it goes without saying, don't forget your passport!