If you still have the tickets to Peru and cannot return them, I highly recommend seeing Lake Titicaca and visiting the floating islands, and the island of Taquile in the lake. There are some nice pre-Inca ruins in that area as well. Lima is an interesting city with great seafood, and you might take a trip to Nazca to see the desert megapaintings there.


However, if you want to spend the time in Argentina, it is beautiful in March. It is pretty easy to get an overnight bus up to Iguazu with full flat beds so you save a night's hotel cost - I took the bus line "via Bariloche" up there last March, and had a lot of fun. You need at least 2 full days at Iguazu, one to see the Argentine side of the falls, one for the Brazilian side. If you do not have a Brazilian visa, you are technically not supposed to cross over, but if you get on the local bus to Foz de Iguazu on the Brazilian side and just "forget" to get out at the Brazilian border crossing, it is ignored.

I also highly recommend Mendoza for wine tasting and great food. Lots of wine tours available, can even bike around the area while tasting. Again, nice overnight buses with beds go there many times per day.

Bariloche in northern Patagonia is a beautiful ski/resort town on an incredibly beautiful mountain lake. Bus and air service there as well, with the "via Bariloche" bus being very nice in my experience.

The easiest way to get bus tickets is to go directly to the counter at the Retiro bus station in BA. Pay cash, as for some reason credit card purchases can not be refunded while cash can. Buses go point to point throughout the country, so you could do a loop, such as BA-Bariloche-Mendoza -(Iguazu)- BA, though that is probably too much for 6 days.

One other tip - if you can plan ahead, the ferry company "Colonia Express" has VERY cheap tickets to Colonia, which is beautiful. http://www.coloniaexpress.com/ar/

I'm jealous of your trip. Have a great time.

Links:
Via Bariloche http://www.viabariloche.com.ar/home/

all bus lines http://www.omnilineas.com/