It's difficult to choose one place to visit in Argentina. However if you have a few days, do not miss the chance of going to the Iguazu Falls. Located in the border between Argentina and Brazil (2/3 of the falls are within Argentina), these amazing 275 falls along 2.7 km of the Iguazu River will give you one of the most spectacular shows of Nature. The Devil's Throat is the most impressive part of it. It's 82 meters high and 150 meters wide cataract.
It is possible to access the falls from the city Puerto Iguazu in Argentina, Foz de Iguacu in Brazil or Ciudad del Este in Paraguay. In Argentina and in Brazil there are National Parks and both circuits are totally worth it, having two different views of the falls. You can also go for a boat ride very close to the falls and to do a "full moon night circuit".
Better to avoid the summer months because it is too hot and humid (remember the summer is December-january-February in South America) although equally beautiful.
Birds and flowers are abundant and colorful.