My wife and I did a few weeks in South America back in 2002 -- went Rio to Iguacu/Iguazu to Buenos Aires to San Carlos de Bariloche to Puerto Montt to Santiago. Our overall trip was a few weeks, but we spent less time in Argentina than you have, so you have time to visit all these places.

If you spend the first several days in Buenos Aires, and then go to Iguazu on a 9:45 AM flight, you should be able to catch the 14:05 flight back the next day to AEP and connect to flights south in Patagonia (i.e. BRC - though not sure if San Carlos de Bariloche is actually Patagonia -- but we liked it there). For Iguazu, we flew into the Brazil side one day, spent time over there, crossed the border, stayed at the Sheraton one night, and then caught a 4 PM flight to AEP during our visit. I had wished for a few more hours on the Argentina side (slept in too late), but if you stay on the Argentina side I think 24 hours there is enough.

Part of our trip was a two day lake crossing from Bariloche to Puerto Montt that was awesome - they also offer a one day version of it (http://www.crucedelagos.cl/index_flash.htm).

Are you locked into returning from Buenes Aires? If not, you could do this crossing and spend a few days in southern Chile before heading back from Santiago (and maybe even get to Punta Arenas via a flight, though that is probably pushing it). It is also possible to connect back through SCL to EZE from any of the southern Chile cities, though that also takes up a good half day of flying.

Another idea to consider -- we also did a one day trip to Colonia, Uruguay, on the high speed Catamaran (http://www.buquebus.com). Makes a good day trip from BA if time allows.