[Cancuk]

3 weeks is such a short time...don't even think about other countries (maybe Uruguay or over to Chile if you're on that side of the country). South America is huge, and going to "check out Cuzco" really isn't practical for a 3 week trip to Argentina. .. that's like saying "check out Montreal" when you're doing a California trip.

Buenos Aires - Mendoza - Lake District - Patagnoia (time permitting/better if you fly down there) - Buenos Aires

or

Buenos Aires - Uruguay (Montevideo/Punta del Este) - Iquqzu Falls - Cordoba/Rosario (optional) - Buenos Aires

My suggestions.