RE: apartment, check out Eastbay1K's signature line.
With such a short trip, I'm not sure I'd do Mendoza unless you are really into winery tours. It's a perfectly nice city, but not a lot else to see.
Iguazu is really unique, though. You'd need two days to see that, even flying, and a Brazil visa to do it right.
If you enjoy a good walk, I found these free walking tours a great way to see a lot of BsAs. Free, just tip what you like.
http://www.bafreetour.com/english-home
http://www.tripadvisor.com/Attractio..._District.html
There's a great street fair stretching from near the Casa Rosada to San Telmo on Sundays. Worth a visit.