Buenos Aires used to be a great city to visit, with a ton to do, good food, and low prices. It's still most of that, but it's not nearly as cheap since the end of currency controls.
I was very lucky to visit Montevideo during the 2025 Día del Patrimonio Celebrating Uruguay's Heritage. A huge variety of locations were open that would normally be closed to the public, and I was able to visit a number of them including the Palacio Legislativo, French Embassy, Piria Palace, and the Montevideo Naval Base.
As a place to visit, there wasn't a huge amount to do within Montevideo itself. There were museums, but generally small. Restaurants were good, but not particularly cheap, and the hours tended to be quite limited (evenings, and maybe 12-3pm).
If Montevideo had limited attractions, Asuncion went a step further. There were still things to do, but I don't think I would have been able to spend more than a weekend on tourist stuff.
Iguazu Falls is a very large system of waterfalls on the border of Argentina and Brazil. It was fun to visit, but taught me that my rain jacket really needs an upgrade and that viewing a waterfall during a thunderstorm does not really add to the experience. Still, there were some breaks in the rain to get some good views, but I'm pretty sure this day gave me a pretty nasty cough.