Mendoza is nice, Iguazú is spectacular. You need at least 2 nights and you can stay in the Argentinian side and still enjoy it. The Brazilian side is also worthy but you would need to apply for a visa. I would take a few days off Buenos Aires and go to Iguazú.
Re hotels, there are many good places to stay in Buenos Aires, no need to pay extra to stay at the Hilton or Hyatt.