mario....Good advice so far about Argentina.

I would give the falls at least two full days...I don't see the point going all the way there just to rush the experience. Bariloche might be a better stop for you in Patagonia rather than go all the way down south if you're not really into scenery. Mendoza is the capital of the wine growing region so if you're into wine then head for there. Salta would also be worth seeing.
Montevideo and Colonia in Uruguay can easily be accessed from Buenos Aires....I would stay in Montevideo for a couple of days and take a trip to see Colonia from there.