I was in South America for my 60th it was the pinnacle of my travel ambitions, I'd always wanted to go there ever since my class covered it in Geography way back when, I think I was maybe aged 11 or 12, before that I didn't even know it existed. We visited Buenos Aires, Argentina, Uruguay, Iguazu Falls from the Brazilian side and finally Rio de Janeiro I feel very lucky to have been there, I will always treasure those memories which exceeded by expectations.
I hope you have a lovely time whatever you decide to do, as others have said that will largely depend on what you like.