I´ve been lucky enough to visit three countries in South America so far, although to be fair two of them, Brazil and Uruguay, were day visits only. Still, I have two new stamps in my passport plus the various exit/entry stamps that Argentinean border control are so keen on, eight in total I think!
Colonia del Sacramento is a small town in Uruguay which dates back to the early Spanish and Portuguese conquests of the 1500s, it only takes an hour or so from Buenos Aires by ferry and is one of the highlights mentioned in both of my guidebooks, I can see why. The town itself is very small and you could easily walk quickly round it in a few minutes if you wanted to, you wouldn´t though because it's so picturesque, with small alleyways running of the main plazas, quaint houses painted in pretty pastel colours and, on the day we went, a bright sun shinining and strong breeze blowing off the river, leaving the silhouettes of trees dancing on the walls of the buildings.
Iguazu Falls is a very large cataract on the border between Argentina and Brazil made up of many waterfalls and is four times bigger than Niagara Falls, which incidentally I've also visited. I have to say Iguazu Falls was much more impressive, simply because there's just so much water! The majority of the falls themselves are on the Argentinean side but you can get a better overall view from the Brazilian side, which is where I visited first. The Devil's Throat (the main waterfall) can be visited from both sides and is phenomenal! Once you get within a few hundred metres of it there's a constant spray in the air and you can hear the roar of the water tumbling endlessly, and if you dare the platforms that jut out over (Argentina) and next to (Brazil) the waterfall, you are guaranteed to get drenched. The view over the waterfall from the Argentinean side is particularly beautiful as you can peer over the side and see the waters falling and forming the mists that sway and change direction with the wind, and watch the swallows that live in the cliffs beside the falls swooping through them. The smaller falls are also very pretty and it's interesting to see the plant life that grows up around the rivers and falls, and that you can see clutching on beneath the rushing water.